I’ve often heard it said some people “never grow up”. It is said in a humorous way. My wife may say that about me at times. But, many times it is said in a not so funny way, usually speaking of a person’s continued immaturity.
I have another thought. Maybe for some people it is that they have never had an experience that instills maturity in them. It’s possible.
I’ve personally experienced and observed that there are some circumstances in life that bring more automatic maturity…a sort of forced growing up mentality.