Augmenting Katze's already clear answers above, I would like to add the following. 1. Grown-up as an adjective may be used to refer not only to chronological age or physiological development, but also attitudes and ways of doing things. Example: John and Mary have been really grown-up about their divorce: no drama, no bitterness, no recriminations. They have remained friends and plan to raise their two daughters together. They are even considering moving into the same house again. 2. Grown and grown up: the difference Example: Your baby has grown so much since I saw him three months ago. (The baby is 9 months old and it certainly has not grown up yet!)

"grown-up" could either be a noun or an adj , depending on the context , some spell it as" grownup" --> as a noun : I have to talk to a grown-up ( I have to talk to an adult ) this movie is only for grown-ups ( this movie is only for adults ) --> as an adj : my daughter helps me to do the housework , she is all grown-up now " grown up" is the past participal of" to grow up" --> ex : the girl has grown up ,she doesn't want to play with dolls anymore

** not grow's past participle.