I think each person & each system is different & everyone has their different needs, but I can speak from my experience: I am a 33 years old woman but I have child parts. the Question for me is when & where the child parts can express themselves. I try to limit their expression to therapy, so eventually it comes out as my child parts interact with my therapist quite a lot. Now that I am on a medical leave I find my teenager parts playing computer games a lot & wanting to stay home alone all day long.
So in my humble opinion it is not a question of wither or not it's ok for a grown person to act like a child, but when & where it is possible to do so.