Look for Al's guides in the guides forum, to learn about training efficiency. Any player does gain %, but it slows down at an unknown rate, probably every few seasons of age. However, we have examples to demonstrate that if they stay effective, age and stars don't tell the full story. You decide who is in your 11 and how long.

Couple seasons ago subbed in a veteran ML player, probably 60%Q, (natural dml) who was unplayed for a few seasons, came in scored 3 goals in maybe 20 minutes, rated a 10, took MoM.

Just yesterday I played a match where 3 defenders from 37 to 66 rated higher than my usual best DC of over 100Q. Anything seems possible, many strategies will succeed.