Number + 岁
岁 (suì) is a measure word used specifically for counting years of age. It follows a number to express how old someone or something is, and also appears in literary contexts to mean "year" in general.
我女儿今年三岁了,刚上幼儿园。
wǒ nǚ'ér jīnnián sān suì le, gāng shàng yòu'éryuán.
My daughter is three years old this year and just started kindergarten.
爷爷已经八十岁了,身体还很好。
yéye yǐjīng bāshí suì le, shēntǐ hái hěn hǎo.
Grandpa is already eighty years old, but he's still in good health.
你家的猫几岁了?
nǐ jiā de māo jǐ suì le?
How old is your cat?