I'm a bit interested in The source of the statistics here. Is it the app store?
In that case, I think the "earlier" category is way bigger than 4%.
My dad hated iOS 6 so much I had to get him a non-upgraded iPhone 4 so he could continue on iOS 5 after I convinced him to upgrade his previous phone... And he is absolutely never on the App Store and has no apps installed whatsoever. My mum has iOS 5 on her iPad 1 and is not using any apps outside the built in.
I do believe that a lot older people have no idea about upgrades and no idea about the App Store. I peek at people's phones at work and after passing 50, the amount of apps on your phone seems to dwindle and at retirement age, it reaches zero third party apps.

Of course, the developers are hardly interested in the users not caring about the app store, but these users are still using their i-units with older versions of iOS but it doesn't show.
It would be interesting to see actual numbers in that statistics and compare to official sales numbers over the years.