How Old Should a Kid Be Before They Listen to Rap?

Rap has been one of my favorite genres to listen to for many many years. Some of my favorite artists include artists like Drake, B.O.B, Big Sean, Wiz Khalifa etc. I love the catchy beats and the catchy lyrics on most of their music, but it's funny how dirty a song sounds when you hear a little kid singing the lyrics to one of your favorite songs. I have a total of 8 nephews and nieces. The three oldest are 12, 9, and 7 years old. They absolutely love the same artists I do plus artists like Tyga and Lil Wayne. As an aunt, when I hear my nephews singing Lil Wayne songs I get a little disturbed because of the lyrics in the song, they have no clue what they're singing and what it means!…Or do they?! I don't know if they do or don't but what I do know is that it doesn't sit well with me. So how old does a kid need to be before rap music is ok to be listened to by kids? If it were up to me I'd say age 21 or NEVER! What do you think is a proper age for kids to begin listening to rap music?