Refrain from childish behaviour (3,4,3)

I think it could be either:

Which length description are you looking for?:

3,4,3

10*

act ones age

(Other definitions for act ones age that I've seen before include "stop being childish" , "appear quite grown-up" .)

I've seen this clue in the Thinks.com.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.