There is definitely an overlap area between < 13 year olds that are responsible and > 13 year olds that aren't responsible, but how can you tell who is who? Maybe 13 was a line in the sand to make the law practical.
Perhaps we should follow the example set by statutory rape laws: children of age x (where x<13) can only give their personal information to sites run by children whose age is within 24 months of x.
(Yes, I am joking.)