Where is the line between stereotypes and prejudice?

Stereotypes don’t exist for no reason at all. Then again, stereotypes make people more likely to be that way. They also hurt people who have bad stereotypes, because people assume they are less than they are.
Teenagers are encouraged and allowed to act immature and foolish, and not to be the young people that they could be, because people say it’s “normal” for teens to be stupid.
In general, people are more wary of black people than white people, partially because they happen to be the people who have been living in poverty in America for years, and therefore getting involved in all kinds of other junk.
My Uncle, who is a black police, said, “I love my people, but they are mean to each other.” He’s not prejudiced, he’s just a cop that knows reality and has certain initial assumptions of people based on their appearance.

Where’s the line between stereotypes and prejudice?