Infantilism

I really have no idea what my finger will fall on when I close my eyes and spin the dictionary around and put my finger on a word.  This time it fell on the word “infantilism.”  It means “the persistence of infantile characteristics or behaviour in adult life.”
There is a famous reference that is often quoted “When I was a child, I spoke and thought and reasoned as a child. But when I grew up, I put away childish things.”
I was putting out kids toys for sale yesterday and a few of us had a laugh about getting them in adulthood for oneself.
No one expects a child to exhibit adult behaviour.  When I wanted to show mum the track I had made for all my toy cars that was acceptable as that is a child’s world.    Parents and grandparents find themselves doing stuff they would not do as an adult because to relate to a child childlike behaviour is entered into. 
I would be seen as weird except in exceptional circumstances (ie as a sales rep for kids toys) if I made a track now and showed off my toy cars. (Though I must admit I have a couple of toy cars in our car port so I can say this is a 4 car car port.  When someone saw a picture of it they suggested that was downsizing)
Yet I must admit there is inappropriate behaviour I can show that is infantile.  It is mostly because I learned as a kid to react a certain way.  It may not be appropriate now, but I can still do it.  People throw tantrums as kids.  Some never grow out of it.  The tantrums may be more sophisticated now, but that is what they still are.  Kids write on walls until they learn otherwise.  I doubt I would be invited back if I did that now.
There comes a time when I need to be responsible for my actions.  Being an adult does not mean I need to look like I am continually sucking on a prune.  I personally do not mind being child-like but if I am childish I think it is a problem.