年轻 (nián qīng) describes a person (or occasionally an animal) who is in the young stage of life, roughly teen to middle age. It differs from 小 (xiǎo), which can also mean 'young' but usually refers to very small children; and from 青 (qīng), which is more formal and used in fixed terms like 青年 (young adult). 幼稚 (yòu zhì) adds a negative sense of 'childish, immature'.