Sony has added the ability to upgrade your sub account to a master account once you turn 18.
Sony Network Entertainment chief Eric Lempel’s post on the PlayStation Blog describes how the new feature is available through the website right now with plans to bring it to the PlayStation 4 in the future.
Sub accounts allow minors to have their own PSN account with limits set by parents and guardians for messages, monthly spending and gameplay broadcasts. A master account removes those limitations and allows you to take your profile, friends list and trophies forward.
For details of how to upgrade your account, head over to the PlayStation Support Site