At what age?

At what age do you think it's ok for your kids to have there own Facebook account? I have some nephews on facebook that are around 11 and one at age 6. Today I log into my account and my sons friend who is 7 has friend requested me. First why does he want to be friends with me???
I have not responded to his request at this time, I'm pondering the reasons why he wants adults as friends. If I was this kid my first thought would to look for classmates and such, if you couldn't find certain friends ask them at school to join.
So anyone out there reading this blog post, let me know your thoughts. I am slightly considering my son to get an account, but under my supervision. It would really be more for games and to keep in touch with family for we don't have any in CO.