With this
thread, I found an answer to your question.
Avatars were disabled due to some kind of exploit back around October 2013.
They were
re-enabled in 2015 on April 1
st (it wasn't a prank lol) and also the Full Member requirement was introduced.
So regarding the UID range my educated guess would be 1 - 155000
So beyond this date October 2013, all members can able to wear avatars. So they're here for a long time.