When a sub account holder reaches the age of 18, their account does not automatically become an independent master account. Any purchases continue to be funded from the master account as the sub account does not have a wallet attached to it and Parental Controls can only be changed from the master account.
The sub account holder has the option to upgrade their account to a master account once they reach 18. This means that they can take advantage of all of the features of a master account including all Playstation Network social features and adding their own card details. Friends list, trophies and purchases made on the sub account transfer to the new master account.
Please note the following before upgrading your sub account:
Sign in to the sub account using Playstation Network sign in details:
Tap [PlayStation Store] on the home screen and go to [Online ID] > [Account Settings].
Go to PlayStation Store. Sign in, go to your Online ID at the top of the page and click [Account Settings].