I can see that. It is probably a balance somewhere in between the two extremes.
As a fan, and more specifically a college basketball fan, I would like to see some type of higher age limit on the NBA. Would that be the best thing for the players themselves? Probably not, but it would greatly increase the viewing pleasure for me while following both the NCAA and the NBA varieties of the game.
A 21-year age minimum would keep many in college for at least three years, strengthening both the NCAA and NBA games as a whole.