Not all iOS 9 apps will run on ageing Apple devices, and here's why34
Not all is peachy, however. In addition to all the feature cutting bonanza that iOS 9 for iPad and iPhone Old is about to face, it appears the Cupertino gang will be letting developers tag their apps as compatible with specific Apple chips only. If any app has, say, the 64-bit A7 chip mentioned as a minimum requirement, you are welcome to try running it on the iPad 2. We're giving you 8 million years before you give up in despair.
Thus, this is the first time in history where most Apple device owners will be able to upgrade to the latest iOS, but won't be able to use all iOS 9-compatible apps. Yes, certainly not a great piece of news for what has been a generally upbeat Apple news stream lately. But when you think about it, if a certain app performed poorly on your ageing hardware, you probably wouldn't want to use it anyway. And for developers, it's certainly a more effective approach than peppering their app descriptions with "WARNING: This app won't run on your 4-year old iPad!" type of language, then dealing with angry users who didn't bother reading the message.
Hence, if the App Store deems an iOS 9 app incompatible with your device, it will simply hide it from you. You won't be able to buy the app in error, or get your experience compromised in another way. Said otherwise, Apple is doing this for The Greater Good (tm).
source: Digital Trends