It doesn't really matter to me. I know people that use nicknames since childhood and those who changed to the full name when they were older. Both work for me and tbh most adults that go by nicknames/diminutives have normally only gone by their nicknames so it would be weird to change just because they're older.
*~Loads of Randomness~*
My PNLs:
http://www.behindthename.com/pnl/150423