It all depends on programmers' dedication on supporting all OS versions. I, myself, support all of them, starting with 4.0+, in my numerous AppStore apps - even armv6 ones like the iPhone 3G and the iPt2G running at 4.2.1. (Too bad it's no longer been possible to "hack" an armv6 slice into app archives' executable binaries since middle of Sept so, since then, I was forced to release armv7+-only apps...)
Given that it only takes about 5-10% of my total time to make sure a new feature / dialog etc. I implement works on older iOS versions (including making workarounds - say, when styled HTML text is needed, I just display unstyled ones on pre-iOS6 devices - the small degradation in user experience is still overshadowed by being able to run my app at all), I say devs NOT paying attention to old iOS versions are plain lazy.
Of course, this doesn't apply to stuff simply not possible in early (earlier) iOS versions because of the complete lack of both API support and workarounds; for example, manual camera settings.