There are no big secrets or miracles. There are only ways to maximize training.
The younger a player, the faster the player will train. Young players make for the best pure trainees. That's why 18 and 19 year olds are such valued commodities. That doesn't mean older players in their early 20's aren't worth training, though.
You'll also want your trained players to get 48+ minutes in the position you're training per week. Getting 48 minutes is crucial.
Another thing of note is that guard skills like passing, ball handling, outside defense and maybe driving and jump range are trained faster for shorter players than they are for bigger players. You'll want a 5'9"-6'4" player preferably, though small forwards can flourish in fine capacity at a wider range of heights. Likewise, big man skills like rebounding, inside defense, inside scoring, blocking are trained faster for bigger players than they are for smaller players. You'll probably want a 6'9"-7'5" player here.
I think jump shot is unaffected by height. I don't know about jump range. Stamina and free throw training is unaffected by height; I also think stamina and free throws are unaffected by age, but I'm not positive.