Having an adult-like appearance, manner, or attitude; seeming mature or grown-up.
彼女は年の割に大人っぽい話し方をする。
She speaks in a manner that's mature for her age.
そのワンピースは大人っぽいデザインだね。
That dress has a grown-up/mature design.
最近、息子が急に大人っぽくなってきた。
Lately my son has suddenly started acting more mature.