I think more important than age is signs of improvement. If a player significantly improves, that is a good sign. If a player improves each year, that is a good sign.
Players who are borderline prospects who more or less stay borderline at ages 21-23 probably are what they are. Players who improve from age 19-22 are just showing the improvement in college that everyone would hope they would be making in the nba. They are also demonstrating a discipline and desire to improve, which is crucial for the rest of their careers.
I've also heard a couple nba players say it's easier to get better once you are in the nba (i think scal may have said this. someone else too).
Buddy Hield is going to be good and we all are going to wonder why he wasn't drafted higher even though we already know before it even happens.