I have googled around for the youngest 
Facebook user in the world, some registered their 3 days old baby and some even made an account for their unborn child. That obviously doesn't count because those babies can't even tell what a mouse is let alone update their statuses.

My 6 years old son on the other hand is a true blue 
Facebook user since, well yesterday. I created his account last year but he was too engrossed with Ben 10 and 
Spongebob to care. Flash forward today, after learning to watch music videos on YouTube and playing some games from 
Miniclip, Adam started to actually use 
Facebook.

He types his own status, approved his friends request, comments on people's updates and even chats with his uncles and aunts. Yesterday he spent over 2 hours with the computer using 
Facebook.
Okay now I only have to teach him some basic 
CSS and 
CISCO programming, then he'll be ready to take on the world.