Effective immediately, you can now upgrade your PlayStation Network sub accounts to master accounts. This long requested feature will allow you to upgrade your sub accounts through the PlayStation website and coming later to the PlayStation 4 itself.
In case you aren’t familiar, sub accounts allow users under the age of 18 to access PSN while letting parents or guardians manage their online experience. A variety of PSN features can be limited through sub accounts including messages, monthly spending limits, and gameplay broadcasts. Additional information is available on our Knowledge Center, here.

If you were on a sub account and turned 18 (Happy belated birthday!), you may want to upgrade to a master account to access the full feature set of PlayStation’s online experience, including PlayStation Store. Your profile will be carried over when upgrading to a master account, along with items like your friends list and Trophies.

You can upgrade your account at the account page at the Sony Entertainment Network website. They also set up a small FAQ page that should also help out on this move.