I'm a grown-up or I'm grown up Both are correct. grown-up is a noun “grown up” is a phrase functioning as an adjective. It’s similar to saying “I’m a woman” or “I’m womanly”.

1. If you want to say that she "is taller now" or "is physically bigger" than she was before when she was a younger child, say this. "Wow, look how much she has grown." To explain that someone has stopped acting so immature this can be said. "He pays his taxes, saves his money in the bank, and no longer acts like a child. He has really grown up" I think you need to grow up a little, and stop acting like a child. "Grown up" can be the past participle of "grow up," but it can also be a noun or an adjective; when it is, it needs a hyphen: As a child, l trusted the grown-ups around me. He thought that a grown-up's opinion should be based on facts. 2) Stuff is a collective noun. The plural form of the noun stuff is also stuff and does not end in an "s" to become plural. Stuff is always a plural noun. it is also a mass noun. There are times to pluralize a collective noun. "Fish" can mean one fish or several fish, but "fishes" usually refer to groupings of different types of fish. In this case, it is a pluralization of the collective form. It is not just simply making a "single noun plural". In addition to being a collective noun it is an uncountable noun. ( A mass noun.) Gold, silver, rice, water, smoke and cement are all mass nouns or "uncountable nouns". "To stuff" as a verb can mean to fill a space tightly with something, for example. "He stuffed his wallet full of cash, and ran out the door." "Every day she stuffs that good luck charm in her pocket and thinks positive thoughts." As a verb "I stuff", "you stuff", "he stuffs", "she stuffs", "it stuffs", "we stuff", and"they sfuff" would be correct. "My parents would not allow me to watch Korean dramas, Korean movies, and Korean shows. They wanted to raise me without these kinds of things."