I think if next season we have to choose between 10k 19 year olds and 10k 21 year olds we would be better off going with the 10k 19 year olds as they will continue to train faster and get even better as the season goes on, they will gain experience and their owners will be encouraged to keep on training them.
the difference in speed is minimal. In my opinion, criteria is first how the manager trained them and kept them in good shape, telling the other "hey guy you have the big opportunity to train your player better!"
Experience is not added by NT matches (or my Scalambrieri should have been skyrocketed)