With iOS 9, developers can cutoff younger devices in a way that was not previously possible. Although iOS 9 runs on every device that runs iOS 8, app developers are free to specify more restrictive compatibility requirements.
In fact, with iOS 9, developers can choose to make their apps exclude any non-64 bit architecture. This means all iPod touch models, all iPhones before the iPhone 5s and all iPads before the iPad Air will not be able to install apps where developers have required 64-bit CPUs.