In response to the 6th post I would like to add that I have known people who have been forced to grow up too early. Then, later in life, they regressed. I think that this really relates a lot to experiences that people have had in life and how they reacted to them.
It is kind of like the phrase "you are only as old as you feel", growing up is not the same as aging. Some people are forced through life circumstances to "grow up" well before others.
A personal observation of whether one's own self has "grown up" is absolutely a state of mind. I know many people in their 50s (including myself) who feel as if they have never grown or matured to a degree where they consider themselves a completely serious, responsible adult. After all, what better times can a person spend than during their young years? I certainly remember mine fondly and try to live in a care-free manner (at least somewhat so) still today.
Perhaps you are asking whether growing up is a state of mind. Surely it is. Growing up is not so much a matter of growing older, which we all do.
Growing up involves changes in one's average state of mind, if you will. We still have changes in our state of mind as Post #2 says. However, we tend to think differently as we grow up and that is, to me, the most significant part of growing up.
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The phrase "state of mind" refer to the the condition of the mind in terms of feelings, emotions, thoughts, which changes from time to time under influence of the immediate experiences of the person. It is a temporary state of mind which is nod distinguishable from things like mood or temper. It changes and fluctuates every day, every hour, and may be even more frequently. Thus there is no single frame of mind one grows up with.