I have a mac with OSX version 10.5.6 and I'm wondering about the limits of developing iPhone apps with it.
Since I need to install an old SDK, will the apps I make still be possible to use on new iPhones? What kind of features will I miss out on, by using an old SDK? Are there any major changes in the new version of Xcode that will make it reduntant or unprogressive for me to learn an old SDK?
You will not be able to submit an app to the App Store unless it has been compiled against the latest iOS SDK (iOS 5.1 at this time, iOS 6 soon.) This does not mean it needs to use the features in the latest SDK, but you must have used that SDK when compiling. Apple will automatically reject apps compiled against an older SDK.
If you don't care about the App Store and just want to play around, you can do so with an old SDK but it might not allow you to attach the debugger to a device running a newer version of iOS.
Since I need to install an old SDK, will the apps I make still be possible to use on new iPhones?
The code is for the most part is still the same. Your apps will work on all iPhones, iPads, and iPod touches.
What kind of features will I miss out on, by using an old SDK?
Faster Compiling, Code suggestions, and a few other things that won't prevent you from making iOS apps in the future.
Are there any major changes in the new version of Xcode that will make it reduntant or unprogressive for me to learn an old SDK?
No
Good luck!