I have never had any problems with installing any app on my iPod 4. But now the latest 5 apps I have tried to install I have got an error that tell me I need iOS 7 or later to install them. Why do so many devs restrict lower iOS? Is there some new features that are only available in iOS 7 and up?

Only 4% of devices do not have iOS 7 yet, so there's not much reason for spending any extra work on supporting older versions.

https://developer.apple.com/support/appstore/

Another factor is that there's no way in the AppStore of excluding devices that you don't want to support for performance/RAM reasons.

Say you don't want to support iPodTouch4s because they don't have enough RAM, you can't enter that choice into iTunes, you have to say things like "This app needs a back-facing camera" or "This app requires iOS7".

It may very well be the case that these apps didn't particularly want to exclude iOS6 as an operating system, they just wanted to exclude all the older devices with weaker hardware and less RAM, and targeting iOS7 was the most convenient way to do so.

A lot of the API changed with iOS 7. While it generally is pretty trivial to support the old way and the new way I assume that some developers don't think it is worth the effort to do so. At some point you just have to decide it isn't worth supporting old hardware anymore since supporting all of that just creates more points of failure. C0lumbo's point is very valid as well if your app needs a lot of resources (older devices are pretty weak and the program doesn't get much memory to work with).

Hmm I only have an iPod 4 and that only goes up to iOS 6. Is there any site where you can see how big the market use is for each device? Is it really only 4% using this old devices?

Hmm I only have an iPod 4 and that only goes up to iOS 6. Is there any site where you can see how big the market use is for each device? Is it really only 4% using this old devices?

Yes, Rattenhirn already posted it.

Only 4% of devices do not have iOS 7 yet, so there's not much reason for spending any extra work on supporting older versions.

https://developer.apple.com/support/appstore/

What you observe is just a symptom of the manufacturer trying to sell new devices every year. Read http://en.wikipedia.org/wiki/Planned_obsolescence if you'd like to know a bit more!

What you observe is just a symptom of the manufacturer trying to sell new devices every year. Read http://en.wikipedia.org/wiki/Planned_obsolescence if you'd like to know a bit more!

Nonsense. You can say what you want iDevices (locked down, lagging in features, etc) but they are far better than Android at supporting older devices. Each new iOS version is typically supported on devices up to 3 generations previous.

Each new iOS version is typically supported on devices up to 3 generations previous.

Sure, but at the rate Apple release new generations, that's still not a long time.

Sure, but at the rate Apple release new generations, that's still not a long time.

It's 3 years.... an eternity in smartphone terms.

Almost every iDevice is on either 7 or 8, whereas Android is much more fragmented (although the situation has improved significantly in the last year or so).