She was the last one on that ship to go crazy.
Ah but those are the ones you have to watch..
I think we are coming at this from two different angles. I am accepting what I see on the screen as the facts of what happened to that character and looking for an in-universe explanation for that which seems out of character. Really any time someone goes nuts and their family say "he was the nicest man! he was never any trouble!" an explanation is looked for. I put it down to the insanity of Ocampan adolescence. Her age is irrelevant as we have no idea what her life cycle will be. As I said, imagine her directing all that rage at Q and it would be naught but a tantrum.
However I accept that you are coming at this from outside the universe and saying No! This is not what should have happened! This is an attack on Kes fans!
I find it more fun to work out a good reason why a dumb character/plot detour happened in-universe. We already know why they happened outside of the universe: bad writing etc..