Turning 18 is great. In America, it means your country trusts you enough to vote, buy cigarettes, and now Sony trusts 18 year olds to shed the restrictions presented a parentally controlled blocks and upgrade to a fully functional master account.
Sub accounts have restricted access to PSN features like purchasing content online, messaging, and mature content. In the past, players who wanted to lose the restrictions without losing their trophies, friends list, and the other aspects of their PSN account had no options. Now users who want to upgrade can do so freely on their 18th birthday as defined by their PSN profile.