As it is right now to have a Facebook account, you're supposed to be at least 13 years old. That certainly doesn't keep some younger kids from getting their own accounts by simply gaming the easily defeated age verification process. Some of the people on my friends list have children as young as seven years old with their own accounts, which have been active for over a year.
Personally, I never saw the reason why younger children shouldn't be allowed to use Facebook. Most of the younger kids I know have Facebook access and use it only to play games such as Farmville. Granted a parent would have to actually parent and be sure the children didn't add anyone that might do or say things that are inappropriate. That would completely rule out everyone I know. Facebook is apparently investigating allowing younger children to access Facebook.
The Wall Street Journal reports that some mechanisms are being tested to connect the child's account to the parents account and allow parents to decide whom their kids can friend and what applications they can use. These features would be aimed at children under 13 years old, and the mechanism will allow games and other content to be charged to the parent account. Much of the cause for concern on Facebook's part over having younger kids on the service has to do with federal regulations requiring Facebook to get a parent's permission before collecting data on children. Allowing access to Facebook for kids under 13 really seems like something Facebook has to do considering many kids have accounts already and Facebook is technically in violation of laws concerning collecting personal data on kids.