Interesting.
As a developer, When Android 6 hit the market, we decided to immediately stop Android 4.3 development(4.4, 5 and 5.1 are similar), that time it was only 3% using Android 6. We knew most major devices(Chip, hardware...) can support Android 6. We release the application with less then 25% support, now it has grown to 48% and we made the right decision(Less bugs and more universal). My Device bought in 2013 support Android 7. It's the Manufacturer fault for not updating software.
Android 7 has a good security module and dangerous if you rooted. We want to port but people are stuck with Samsung devices(Worst software support, I own 2 devices myself and it a pain getting upgrades). I love android, but developing for Android is a pain, Apple development is a breeze. If we start developing, target the Latest apple, after 1 year more than 70% are already on it.
On a side note, it time to upgrade your children devices!