About 80% of users are on iOS 5.0+, so, 20% are < 5.0. But the 20% that are < 5.0 are not the ones that will be downloading and installing fancy new apps. If they were big app consumers, they would update the OS.
The iPhone 3G is stuck on iOS 4.2.1. When I still had my 3G, I happily bought apps if they still ran.
We also had quite some support requests for a three digit dollar app (!) because customers never bothered to update their iPad, the OS versions were all over the place.
