To appear or behave more grown-up than one's age; to seem mature or adult-like.
彼女は同い年の子よりずっと大人びて見える。
She looks much more grown-up than children her age.
その若い俳優は大人びた役を自然に演じた。
The young actor naturally played a mature role.
子どもが急に大人びる瞬間を見た。
I saw a moment when a child suddenly seemed grown-up.