Quite simple really.
For full season ball, Low A (Lansing level) age 21 is borderline about to be irrelevant, 20 is good, 19 is better, 18 or younger is exceptional.
Advanced A (Dunedin level) 22 is borderline, 21 is good, 20 is plus, 19 is exceptional
AA New Hampshire level, 23 is borderline, 22 is stillgood, 21 is plus, 20 or younger is exceptional
AAA 24 is borderline, 23 is still good, 22 is plus, 21 and young is exceptional.
These are the guidelines I use for regular players in full season leagues.
Short season leagues are a bit harder to use agelevel because since they start up after the draft, there's always a bunch of new players that are probably better than that but end up there because there's nowhere on full season rosters to put them, and for pitchers because they've already pitched in the college or high school seasons and teams pt them on limits.
But...
Generally speaking, rookie ball (GCL)I'd want them to be 17 or younger to be plus. 18 is still fine, 19 is pretty borderline unless it's their first season
Advanced rookie ball (Bluefield) ablut the same as GCL except 17 or younger would have more shine to them.
For sort season A ball, (Vancouver) ... pretty much the same guidelines as Low A.
Then of course, you also have to consider results and how many years they've been in pro ball already. Like a 21 year old Cuban raking in Low A would still be something to keep an eye on if he's in his first year in pro ball, vs a 21 year old in his 4th season of ball.