I just want to clarify that this isn't tied to maturity : you can always change your own diapers. Instead, it's an area lock that prevent changing in some areas. What can occur,though, is a bug that incorrectly clear the "allowed to change" flag when you exit. In that case, a simple fix should be to go to your home, save and quit, then reload. This should clear any incorrect flag and allow you to change again.