After a couple of seasons it will be difficult for most 18-19 YO to start for a team in league games, if the team wants to win games. With only 5 players on the court, it is hard to hide a weak player. So that means you are going to have to be training some players in their early 20s, and maybe use the youngest players in the scrimmage.
As time goes on, the age at which a player is considered trainable will continue to increase.