The reason for this is COPPA. Specifically it requires creating an orwellian audit trail of all personal information, or anything that might be construed as personal, or deemed personal, or contain content that is personal, of a child's activities. Basically Microsoft has to log everything every child on it's service does and email it to parents if at some time in the future they ask for it. Like every other provider they just say it's too hard and demand you tick the 13+ checkbox.
COPPA is the opposite of child protection and should be abolished.