I posted this on a thread earlier, I just want this on record for future references.
For every LeBron, KG, Kobe, etc, you have 50 guys who came out of high school you've never heard of and were never drafted, only to have their lives ruined. Slimball agents are filling their heads with lies that they're good enough to be 1st round picks, so they sign with the agent and are now ineligable for NCAA play. Obviously they're not NBA material at the time, so they go undrafted, and can't play college ball. Since they're usually not smart enough to go to college and get an education, they are now screwed. Obviously by going straight to the draft, they think of it as a quick pay day, and a way to "get out of the hood" the easy way.
So now they can't play NCAA, they can't get into post secondary school, and they can't make the NBA. You're only choices are to go through life working at McDonald's, go into the crime business, or abandon your family and give Europe a shot.
By making the age limit 20, you're giving these kids free education which most of them will end up using, and a chance to take baby steps in developing their game.
I bet Sebastion Telfair and Gerald Green wish they had to college for a year or 2.
P.S. Just for th record, I don't care if the age limit is 19 or 20, both are fine with me.