it depends on how many green do you have? 22-25 yo the speed is the same and if next year you choose an other player to train the 22-24 year is the same not important.
if you want to play with a 7* team. train scout if you have a good one(right SA, good multi position).
PT young only when you can't find a scout in his position or you want 3 positions player. Even 9* player good young one can't beat a good scout one and you save time to train him.
some of my friends have a tatic to train legend player, this legend is the only one in his team, other is 6* and low 7*, he will be man of the match of any match and it took not much to keep him as legend. in the case he must take freekick, conner, penalty and must be an AM or ST