年纪(nián jì) - age, years (of a person) in Mandarin

年纪 (nián jì) is a person's age. It is a noun, not an adjective, so Mandarin does not say that somebody "is" 年纪 - it says their 年纪 is big or small. 他年纪大了 is "he is getting old", and 年纪很轻 is "very young". Nothing else in the sentence has to change.

In speech it is the warm, conversational word. 您多大年纪了 is the polite way to ask an older person their age, where 年龄 belongs on a form and 你几岁 is for a small child. 不在乎年纪 or 年纪不小了 turn up constantly in the sort of family conversation these stories are built from.

The trap for an English speaker is that "age" in English also covers eras: the Ice Age, the age of steam. 年纪 never does that job. For a historical period you want 时代 or 年代, and reaching for 年纪 there produces a sentence about how old a century is.