You must set up your own master account first, and then open a sub-account for your child. If your child is under 18, he or she may not open a master account. There are additional requirements to complete registration of the sub account for a child under 13. Please follow the specific instructions provided during your registration. You may not open a Playstation 3 Network account for someone who is not your child or for whom you are not the legal guardian.

Source: Sony