The Omnitrix DOES turn Ben into versions of his aliens that are the same age he is, though. The episode where he and Max get aged down proved this.
Outside of Kevin's backstory becoming an ungodly mess, what other retcons could you mean?
Alright Omnitrix, give me Humungousaur!
Nolan Long
Jonathan Long
It seems to transform into the "equivalent" age whatever species you're transforming into
Parker Jenkins
How the fuck do you have literally no idea how the omnitrix works after all of these years
Austin Martin
For example magic was a lot more mystical in OV. I don't know why that team had to emphasize on it when only AF was like that and in UA it was shown the more mystical elements of magic.
Also there's no more human alien hybrids which means humans no longer have the coolest superpower present in any fictional setting in UAF we can fuck good and we're relative versatile enough that there's a wide range of aliens we can have kids with.