In theory, you have to be 13 to be on Facebook. In reality, it's not too hard to fudge your DOB to get on Facebook, and some parents assist their under 13 y.o. children in having an account.
All that said, I haven't personally run across any five year olds on Facebook. :seeya:
Yes I agree. It just depends on what parental supervision is going on, that's really all there is to restrict a kid from getting a Facebook. Which is why I was thinking for a moment that AH 5 y.o. Son may possibly have a FB! Maybe a way for him to brag about how many detentions he has got in kindergarten or something. But I guess people were not actually saying he had a Facebook.