You're right that there are 25 year olds at high-A just to fill out a roster (and maybe get them started toward a coaching career), and AAA is littered with 30-somethings still hanging on. Silly little rule of thumb I use: to reach the majors at age 25, on a team that runs you up one level a year, you ought to be 24 at AAA, 23 at AA, 22 at A+, 21 at A; I don't pay a lot of attention to age 20 and younger except to see that they are making some kind of progress. If you're older than these numbers, you're old for your league, regardless of the averages. This doesn't give college players a fair view of course - but they enter the farm system a little more polished, so I adjust as I see fit. And guys who are signed really young, I just don't start the clock until they reach Elizabethton or higher. It gives me a feel for whether a guy is on track to make it. There are exceptions but players who don't reach the majors by 25 are usually marginal. Feel free to hate this rule, or modify it, or start fresh with something better.