There is some mention of 21-24 as the next age bracket, but that's anecdotal and old. In a few months I imagine Al will have good data for learning rate, TEF, over ages. He probably has a guess at this point.

Anecdotally, I have players over 25 who can pick up 3 skill % in a session. My MoM last match is 26, my best player this season is my oldest, 27. Some say 18-19 rate erratically, some say GK are best older, or ratings stabilize. I think the distribution of skill points after a match reinforces the skills that each player used in the match (while our training drills might throw off their balance. Perhaps explaining why fast training doesn't always mean better results.)