It's a client-side flag saying "treat this request as coming from a child (whatever that means to you)". I don't follow what the jurisdiction concern is.
[EDIT] Oooooh you mean if a child is legally 18 where the server is, but 16 where the client is. But the header could be un-set for a 5-year-old, too, so I don't think that much matters. The idea would be to empower parents to set a policy that flags requests from their kids as coming from a child. If they fail to do that, I suppose that'd be on them.