I have seen this process firsthand. When I coached for the Clippers, I had to deal with Michael Olowokandi, a player who perfectly fit the description "talented but uncoachable." At practice, I would attempt to point out Mr. Olowokandi's faults to him, ones he constantly repeated and resulted in lost possessions for the team or personal fouls that sent him to the bench. His reaction to my attempts to correct his bad habits was to take my input as a personal insult and embarrassment. He told me point-blank that he would not be criticized in front of the team. He stuck to his word and, as a result, had very few successful moments on the court playing the way he wanted to play. He took his place on the list of athletically gifted washouts who have been in and out of the league in the past 10 years.
Wow. I mean, there's no guarantee he would've become a special player had he listened to Kareem but to think what could've been if he'd had a better attitude.
I don't entirely agree with Kareem's point about the age limit causing an influx of undeveloped/unproven talent being so destructive. Olowokandi is also a poor example for him to cite in this instance since he was 23 when he entered the league and in Magic Johnson, Kareem played with and won five titles thanks to a player who came into the league at age 19. There have been plenty of washouts who spent three or four years in college and entered the league in their early 20s and it's not necessarily about maturity (though as Kandi demonstrated, a lack of maturity and bad attitude isn't limited to people under 20). Some players just don't have the talent and/or athleticism to make it in the NBA and spending more time in college isn't going to change that.