"Puerile" means childish.

Puerile is not a common word. It tends to have a negative connotation, while silly may not. "You puerile boy!" sounds plain insulting compared to "silly" "The answers they give are plain puerile" would be a good use of the word.

PS : (1） It sounds silly/puerile. (2) You silly/puerile boy! (3) The answers they give are plain silly/puerile.