Is “un-grown-up” a common, or not so common word?

Question

I came across a word, “un-grown-up” in Jeffery Archer’s fiction, “The Prodigal Daughter.”
In the scene, the heroine, Florentyna Rosnovski asks her governess if her father, the owner of a hotel empire, doesn’t love her mother any more:

“Then why has Papa stopped coming home?” “That I can not explain, but whatever his reasons, we must be very understanding and grown-up.” said Miss Tredgold, brushing back a lock of hair that fallen over Florentyna’s forehead. Florentyna felt very un-grown-up and wondered if Saint Joan had been so unhappy when she lost her beloved France. – P75.

The meaning of “un-grown-up” is obvious, but I don’t think I’ve ever seen the word before.

I consulted online CED, OED, Merriam-Webster, and Collins Cobuild English Dictionary at hand, none of which carries ‘ungrown’ or ‘un-grown-up.’

Even Google Ngram shows neither ‘ungrown’ nor ‘un-grown-up.’

Curiously enough however, Kenkyusha’s Readers English Japanese Dictionary published by a leading Japanese foreign language dictionary specialist publisher carries "ungrown" as an adjective meaning “not grown up, immature, or childish.”

Is “un-grown-up” a common, or not so common, or taken-for-granted word?

Answer 1

A quick google search returns a few headlines:

Adam Sandler's un-'Grown Up' looks

Kids books, fun books, un-grownup kids books

Musings of an Un-Grown-Up

Never Neverland - Un-Grownup talk.

Unsuitable for Adults | games for the un-grownup

I wouldn't say this word is "uncommon" but English is very flexible - and adding an 'un' on the front of anything will be easily understood and accepted.

--Ungrown has nothing to do with this word though -- this is simply just "grownup" + "un".