Ok I have just been searching about sub-accounts as my son has one and
would like to play my AC:R multiplayer but the PSN won’t let him.  This
is what I found out and some of the posts were as far back as 2006.
#1 Sub accounts are for minors (17 and under) and managed by a master account (18 and over)
#2
Regardless of whether or not you have turned off your PS3 CONSOLE
parental settings, when playing online, a sub account will NOT be able
to access any content or play games rated above their age which is
determined by the DOB you gave when signing up.
#3 There is no way to
go back and change the sub account holders DOB so that they can play or
access content rated above their actual age.
#4 Sub accounts do not
‘become’ master accounts once the player turns 18, they remain sub
accounts.  When the sub account holder turns 18 all you can do is create
them a master account so they can play online games and then back up
and transfer all their saved games to the new account but none of their
trophies or achievements will transfer over as they are associated with
the PSN user and can’t be transferred.
#5 In some guides to rectify
the ‘I am over 18 and have a sub account but can’t play online’ issue,
it suggests changing the ‘content restrictions option’ in the account
management section of the sub account found in the master account.  This
option is only available in North America.
#6 When you create the new master account for the person, you will need to use a new email address and username.

The
only way to play games online rated above your age is to create a
master account with a DOB 18 or over which is against the PSN terms
& conditions.