The official age for a Facebook account is 13.
Protected by COPPA rights, Facebook is not allowed to accept younger children to uncover their identity on a social website like that.
However, I know of plenty of parents who have allowed their 9 or 10 year old children to have an account. They have set it up, they hide all personal information, they have one picture and they are just on it to interact with friends (who are also added by the parents) and to play these Facebook flash games.
Personally, I would respect the minimum age of the website. Surely, when my mother asked my opinion on my ten year old brother having a Facebook account, I told her it was her decision, that I advised against it, but if she thought he was ready, the least I would want is that I would be the one to regulate the settings and supervise his behaviour (I have more experience with education as well as social media) and she agreed. Truth is, he is almost never on the computer, leave out on Facebook. It's just there because his friends have it, and they act the same way.