Develop Reference

Recommended Minimum Android App SDK

I am making an android app, and am wondering what the industry's thoughts are on supporting older android versions like GingerBread and FroYo. Should a developer like me take the extra step to make my app compatible with those older versions, or are they obsolete? I am speaking in terms of the market in 2014.
Increasing my minimum SDK version opens up some more APIs, so which option should I pick, compatibility or APIs and features?

Most of the Android devices are above the Gingerbread level. But there are still a decent number of those devices out there. You, as a developer, must decide if the number of users who would potentially download your app for those versions of Android is worth the level of effort in developing the app for those versions. For the past year, both companies I've worked at have begun to work on 4.0 and above only and have forsaken the lower versions.
To get the current information on what the ecosystem looks like for Android, check out Google's dashboard
As I write this comment, Gingerbread and below is around 15% of the total population.
2019 UPDATE: Google's dashboard states that only 0.2% of Android users are running Gingerbread (Android 2.3). Around 3% are using Jelly Bean (Android 4.1/4.2/4.3), and 7.6% are running KitKat (Android 4.4).
Generally, companies target a minimum version of KitKat, or SDK 19, for new endeavors. For personal projects, we usually choose Lollipop, or SDK 21, as it brings a number of improvements to the table, such as improved build times.

[2020 UPDATE] You need to base on Android Pie Chart . It is always updated.
here's the link of the dash board:
https://developer.android.com/about/dashboards/index.html

For the record - 2020 April
If you look at the comments, they usually set the min level to 19 since years ago. Right now, it is nonsense (also, some comments are obsolete).
We are looked at the statistics of Google, and they say, "+10% of the devices are KitKat or lower. However, it's tough (if any) to find a company that sells phones for Kitkat.
So, why Kitkat 19 is still popular? Because old smartphones simply don't go away (and they aren't updated).
Then, technically 10% of the market uses Kitkat or lower but that information is mostly useless
From then 10%, how many of them will install your application? Or any new application?. I know a lot of people that are happy with their old-and-rusty phone, but they only use for making calls and watch youtube/browser internet. So, usually, the lower 10% is not part of our target market or the target market of anybody.
Twitter said about it:
minSdkVersion · Nov 7, 2016 19
minSdkVersion Apr 23, 2018 21
minSdkVersion Dec 9, 2019 23
When Twitter says API=23, then Twitter is ignoring 25% of the market. It is a huge number. But again, this number means nothing without analysis. Twitter decided that this number is moot because, of the 25% of the users that use an old phone, they are not really in its market share, i.e., people that don't use Twitter on their phones using an application.
So, for the record and as for April 2020.
API = 19 who are you pleasing? Remember, we are 4 years later in comparison with Twitter.
API = 21 is a moderated number.
API = 23 sounds like a rational number.
For example, I run a windows server, and the default browser is Internet Explorer. Do you know how Internet Explorer works in the real world? It doesn't!. It works with google and a few sites, but everything else is broken. And still, developers "support" internet explorer because of the non-existence 6.97% market share (the users exist, but they are not everybody users but intranet users).
Now, if your application is for a government/public interest, then we should stick with the standard or official procedure, and sometimes it could be around API 15 or 16.

For those who are looking for an opinion to refer to, #minSdkVersion updates their Twitter account when they deem it appropriate to start using a higher minimum SDK version. They claim
The minimum SDK version for the practical, rational developer.
As others have mentioned, it's also worth checking the Android distribution dashboard. The full platform version disribution can be found in Android Studio's Create New Project wizard.

UPDATE JULY 2022
This is what Android Studio shows before creating a new Project.

I'm a developer myself. I would look into this from multiple point of view and I would use several factors to begin a project with a particular sdk as minimum.
Beside the chart, that Google is kind enough to publish, which is really helpful and informative and i believe it represents devices around the world.
These are the factors I would take under consideration:
What's my app all about? What kind of people and what kind of age range would attract. Young people tend to keep their gadgets up-to-date.
In which country I'm going to release my app? Not all countries have the same mainstream when it comes to technology.
Does the minimum sdk affects the time or the resources that are going to be used in the app? If having a high sdk as minimum is inversely proportional with the time and the struggle my developers would face in order to make everything compatible, then I would choose a higher sdk, even if I have to exclude a small proportion of users. Let's face it, this is 2016. Technology is running faster than light. Releasing early really matters. In the other hand, if it really doesn't affect that much, then just choose a version that covers most devices.
Security matters as well. Everyday new issues are being patched. Newer version is more secure.
If the idea is powerful, than I would choose as higher as I could. People will follow. Is the truth.
Many functionalities are being deprecated. And usually in older versions. Upgrading from those can be hell.
Can't think of anything else. But if you want my opinion, I would choose API 19 as minimum.
Cheers.

There were only few (~10) percent of users with Gingerbread to download our application from Play Store, so we have decided not to support it anymore. It was about 7 months ago. Froyo and Honeycomb are dead for much longer time.
We would go with Android 4.0 (API 14) as minimum for new projects. There is no real benefit from supporting older version unless you are forced to do so (client requested it, etc.).

Note the new directive from Google around the dropping of support for ICS (sdk 14 and 15). Recommended course of action is to set your minimum to 16. See https://android-developers.googleblog.com/2018/12/google-play-services-discontinuing.html
Note that existing apps will continue to work ok.

Which version of Android should I use when developing Android applications?

Should I use the latest 4.0 or 2.3? My phone has android 2.3.5 and it's a HTC Desire HD.
Does the runtime matter or will the applications work still?

First you should choose what device versions you'd like to target: http://developer.android.com/resources/dashboard/platform-versions.html
Based on those numbers it makes sense to currently support Android 2.1 and higher.
Also you should read about <uses-sdk> directive:
android:minSdkVersion and maxSdkVersion define the version range to which your app will be available. If device has Android version out of this range then app will not be visible for it.
targetSdkVersion is the vesrion that you are targeting and that you actually tested against. This attribute triggers Android's compatibility behavior: for instance targeting 2.3 will not show the new look on 4.0 devices.

I think this url will be helpful :
http://developer.android.com/resources/dashboard/platform-versions.html

All versions of android are backward compatible which means that an application designed to work with 2.0's api will work on 4.0 so it basically comes down to whats your target customers. there's still a vast majority that uses 2.x (unfortunately) so it is probably smart to develop for 2.1 and up..
but anyway this is a pretty common question and i'm sure google will produce a lot of useful information in the subject.

why is Android API level 9 obsolete?

Android version 2.3 - 2.3.2 (API 9) are declared as obsolete, and according to Android Market statistics, are very rare (0.5% of market users).
The question is:
Why is API 9 declared obsolete and more importantly, is there any reason why should I not use it? I've been using API 9 and see no problems with it... am I missing something?
Better not fix it if it's not broken, right?
EDIT:
Clarification:
If I would update my applications' API from 9 to 10, what would be the benefits?
I can only see negative effects:
1% of users will suddenly find out that the app doesn't work on their phones any longer. This is even worse with paid apps. Or that I'd be forced to maintain two versions of each app.

Have a look at the description of API levels. Gingerbread API (9) has been replaced by Gingerbread MR1 (10). You should upgrade to this API.

The current version of the android OS is 4.0 (Ice Cream Sandwich). 2.3/2.3.2 is quite a few versions ago and was quickly replaced by 2.3.3. (api lvl 10), so it is very reasonable (as time progresses and changes / updates are introduced) to "shelve" a specific version so to speak.
However, the one strength in the android OS is backwards compatibility. While older devices (pre 2.3) will not be able to run your applications, all applications going forward will. There is technically no problem running the older OS's but bug fixes/changes...etc mean you may be inadvertently using buggy code that contains security flaws or other issues.
It's best to pick the API version that will reach the majority of your target market most effectively.
For example, if you wanted to take advantage of some screaming new OpenGL features on the newer higher powered phone you wouldn't want to run it an older API level and possibly create a storm of angry users because their phone doesn't have the HP to run it!
However a more simple application might be fine just running on an older OS (like the Chuck Norris random fact generator).

You should always go for the lowest possible API to use. Simply because the lower your API level is, the more users can you reach.
So looking at the current stats, you are able to reach 97.9% if you "dumb" yourself down to API level 7.
But why API level 9 is obsolete? One could not know for sure, but most likely because it had some major flaws.

The Lvl9 is obsolete because if you want to develop for Gingerbread you should use 2.3.3+ (Lvl 10). This is because the "old Gingerbread" (lvl 9) has some significant issues.

Well, I don't really think you should be worry about the OS itself. But many Droid programers, like myself, are not testing Gingerbread version on emulators. Mostly, because it's not showing on the SDK interface (appears only when you check the obsolete filter). So, I guess that other programers are ignoring this version also. That's the only problem I can think.

What Android SDK should I use right now?

I'm a CS student and with my team we're going to realize an android application for our Software Engineering course. We're not Android developers and we using this opportunity to learn how to do it.
We know it's available the 3.0 version but that basically has tablets as target. Our target is smartphones, so the question is: should we stick with 2.3 or should we use 3.0 API.
I know the natural answer would be "2.3" but it's not really clear to us where "3.0" is going.

I would even go for 2.1 or 2.2 - 60% of the market uses 2.2, 20% 2.1 and 6% 2.3.
3.0 is atm only available for tablets - targeting this will limit your target market a lot, as no mobile handsets will have 3.0 any time soon.
See also this q&a: What version of Android should I develop for?

3.0 is the future, no doubt. But 3.0 is currently just available for some tablets. Almost all smartphones are currently using some 2.x version.
As you can see here, the version 2.1 and above are widely used. So I personally would target 2.1 and above.

Depends on your application's scope.
If you want to aim at as many people as possible, you should go for 2.1, or 2.2, if you don't use any specificity from the 2.3 SDK (NFC for example).
It is important to know that, because only about 10% of Android phones are running 2.3 (and even less I think). More than 70% have 2.1 or superior, so if your target is mass distribution, 2.1 would be nice. (You can develop an app under 2.3 SDK aiming at lower version as well, just add it in the manifest)
Otherwise, if your application is tablet-aimed, use 3.0. Otherwise go for 2.3.

What version of Android is on the G1?

I just uploaded my first app to the Android marketplace and one comments says, "It force closes on G1". What version of the Android OS does the G1 use?

Originally the G1 shipped with 1.0, then upgrades were released to update the phone to 1.1, 1.5 (Cupcake) and 1.6 (Donut).
So you would expect most users to be on 1.6. However, there are a couple of extra things you may want to consider.
Firstly, some users will be running custom firmware. A quite a few G1 users - me included - run community firmware on their devices, such as CyanogenMod. Often these releases are 1.6 releases with 2.x functionality ported from the Android Open Source Project (AOSP) tree, so you could consider them as a hybrid release, putting the phone somewhere between 1.6 and 2.x. I wouldn't worry about the people running custom firmware since they are so many different firmwares and builds of each firmware, and people who put custom firmware on their phone usually expect to fend for themselves. I only mention it in case you get some weird bugs you really can't replicate which might be caused by this.
Secondly, I'd recommend looking at the Android Device Dashboard as this shows what fraction of users of all phones are running each release:
At the time of writing 31% of users are still on Android 1.5 and I would guess quite a few of these are running G1s.
So my advice would be to test your application on 1.5 and 1.6 if you want to support G1 users.

The latest should have been Android 1.6, but originally it was shipped with Android 1.0 (Source).

The latest update they've pushed out to the G1 is Android 1.6. They're supposed to be working on trimming down 2.0 to make it fit on the G1 but so far it hasn't been available without hacking your phone.

Currently 1.6 (Donut), this is most likely the last OS version that will be on the G1.

Originally it was shipped with Android 1.0. Now it uses Android 1.6