Until you get into D.I or D.II, I would only suggest training until around 23-24 years of age, otherwise you'll pack on salary to a player that you won't be able to afford.
I think a good model (personal preference) is generally to have two main trainees and train them until they are 22-23, and then look at picking up another set of young trainees to become your primary ones. Then you can start training your younger players, while still polishing off your older ones for training weeks that allow you to have more than 2-3 players trained (such as 1v1).
I've always noticed a pretty large drop off in training speed when players get to 24ish years of age, but when they aren't your main training focus it is nice to see them pop here and there and continue their growth while you get the new guys studded up.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs