boyish
adj.having qualities that are usually associated with boys or young men. It can describe a person's looks, behavior, or a style that is simple and not too formal.
adj.possessing characteristics, features, or a style typically associated with boys or young males. Often used to describe a youthful, unassuming, or slightly masculine appearance or demeanor.
“He has a boyish smile that makes him look younger.”
“She wore a boyish haircut and oversized clothes to keep a casual, relaxed look for the weekend.”
“The actor's boyish charm was a significant factor in his early success, as it allowed him to play a wide range of youthful roles with ease and authenticity.”
From boy + -ish.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.