You can still download apps for iOS 3, 4, and 5. iOS 6 will be safe for a while.
As far as I know, it hasn't been that long since Apple required that app updates drop support for versions of iOS older than 4.3.
Yes, the current Xcode 5, last updated in April 2014, will still let developers design and build iOS 4.3 apps.
Now, if an individual app developer decides their new app needs iOS 7.1+ or iOS 8.0+, there is nothing you can do about it.