Normal English would say, “What is a grown up?” “What” indicates a “person, place or thing”.
The more awake you are, the more you know that considering a person to be similar to a thing is a deep, deep problem. It is like throwing away the best aspects of being human and starting work with only what is left over.
So, “who” is a grown up if we have room to address the question as if you and others are not things but … and watch the forced grammar here… “something” else? Our everyday language is weak about “things” that aren’t “things”. Even what isn’t a thing is related to like “something else.”
Let us make a category for a “connected self” — more like a deep network than a historical, story-based person. Against that background, what would a grown up be? (and here I use the term “grown up” in a developmental sense of “adulthood” where the basic important capabilities are at play in a mature way).
Operating as a grown up would put into play — into engagement with others and life, the deepest strengths and insight you have. That would be a grown up way of engaging.
Operating as a grown up would have freedom that comes from releasing past offenses, resentments, comparisons to others, and suppressive notions of what you “should” be as distinct from what you have been given to be.
Operating as a grown up would at some point recognize and act on the recognition that you will have fewer issues and more interesting ones by putting your capabilities and power in service of others and of life and of what “wants to happen” — that is, in behalf of “unfolding life”, instead of being …again watch the usual grammar here… small and self-serving (wrong self), self=protective (wrong self), and self-concerned (wrong self).
Being a grown up entails trusting life beyond having to control it. One gets further by supporting life and other’s fulfillment than by the kind of manipulation that control usually calls for.
Are you still interested in being a grown up, or have I lost you?