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 on this forum can able to wear avatars. So they're here for a long time.
As far as I knew back then when I first started here you need to at least be a full member in order for you to wear avatars. To be honest this is news to me since I there are only few accounts remaining as newbies who are still wearing avatars like what the OP found coz mostly they already have high ranks as of today while others had already retired from this forum.