I'd say he'll be good by 20, great by 22 and Elite by 24. That depends on the way he is trained though, and your deevelopment as a team; you won't be able to sustain him in a DIV or even DIII league once he's 22 or so.
That said, he's an amazing Guard trainee, and guards do have less of a salary rise than Centers. It's going to be hard to find such a player again, and you could at least afford him until 21 before being in DII or not becomes an issue. So if you think you can train him well, I'd even advise you to keep him. If he really does become too expensive, you can always sell him later.