‘三岁’ literally combines the numeral ‘三’ (sān, ‘three’) and the measure word ‘岁’ (suì, ‘years of age’), forming a noun meaning ‘three years old’. Unlike English, which uses ‘years old’ as a phrase after a number, Chinese places the age measure directly after the number without a verb like ‘is’ or auxiliary words — so ‘他三岁’ means ‘He is three years old’. It’s used exclusively for human age, especially children, and appears in contexts like developmental milestones, school enrollment, or medical checkups.

This term is neutral in register and commonly heard in daily conversation, parenting discussions, and formal documents such as birth certificates or kindergarten applications. It’s not used for animals or objects — for pets, you’d say ‘三岁’ only if anthropomorphizing in casual speech, but standard usage reserves it for people. The character ‘岁’ itself is a classical unit of age, distinct from ‘年’ (nián), which refers to calendar years or general time spans.