It depends on the resources I've got and the form and effectiveness of the player over the few seasons I've played him.

I buy 18-19, and anyone who is making a real impact I'll get to 120 when he's 21 years. If he's good, but replaceable, it'll be under 120, we can win without him. I've had a few players higher in the mid 120s when they were clearly special at age 19-20, so they get to 120 then and still have year 21 to win MoM, train with high efficiency, and get back over 120 quickly.

I like having a few, 1-3 starting 11 players who can be fast trained and are known to be reliable starters, in case of wanting to bump team Q up. But if the season is clinched, like I'll win league but out of cup and cl, why bother. I'll hold resources one season to the next until there's a good option for it.