年轻

年轻 (niánqīng) is the adjective for 'young' applied to people — typically anyone from teenagers through their thirties or forties, depending on the speaker's perspective. It literally means 'years light,' i.e. not weighed down by many years. Two distinctions matter for English speakers: first, 年轻 is for people, not for objects — a young tree is 小, a new company is 新, not 年轻.

Second, 年轻 is broader than English 'young child'; a 年轻人 is a young adult, not a kid (use 小孩 for children). The opposite is 老 (lǎo, old). Common patterns: 年轻的时候 ('when [I] was young'), 年轻人 ('young person / youth'), 看起来很年轻 ('looks young'). It's a compliment in almost every context.