Asking about somebody's age
The way you ask how old somebody is depends on the age group the person belongs to.:
他几岁了? Tā jǐ suì le? (used to ask about a child's age)
他多大了? Tā duō dà le? (used to ask about an adult's age)
他多大年纪了? Tā duō dà niánjì le? (used to ask about the age of an older person)
岁 is a measure word for age, it is preceded by the interrogative 几 jǐ meaning how many? 几 is used when the expected answer is ten or lower.
年纪 niánjìis a noun meaning age, it usually implies old age.
All can be translated into English as 'How old is he'.