From now on, PlayStation Network users have the option to upgrade sub accounts into master accounts, according to an announcement on the PlayStation Blog. Currently, accounts can be upgraded through the web here, and in the future it will be possible to do so directly on the console.
Sub accounts allow PSN users under the age of 18 to access online content with limitations imposed by a parent or guardian. Master accounts have options such as controlling messages, imposing monthly spending limits, and disabling broadcasting.
Upgrading a sub account into a master account after turning 18 allows full access to everything that PlayStation Network and the PlayStation Store have to offer. Profiles, including friend lists and trophies, all carry over through the upgrade.