How old are you? is the standard way to ask someone's age. Elsewhere old means not young, but in a question about age it's neutral and works for a child as well as an adult. The answer is I'm 25 or I'm 25 years old.
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How old are you / is he / is your son?; I'm + number (+ years old); How old is + thing (house, car)?
- How old is your daughter now? — She turned seven in March.
- How old are you? — I'm thirty-one, and my brother is two years younger.
- This bridge looks really ancient. How old is it, do you know?
What changed
First versionI am 25 years.
RevisionI am 25 years old.
It can seem that years already says everything that's needed. But in this kind of answer, years doesn't stand without old: people say I'm 25 years old or just I'm 25. In conversation the short version sounds more natural.
How old asks about the age of people and things alike: How old is this building? Forms and formal language use age instead: What is your age? or At what age did you start school? There you need age, not old.
Your turn
— ___ is your sister? — She's fourteen, she's in Year 9 at school.
Show the answer
How old — The reply gives an age, and English asks about age only with How old. How many years is your sister isn't how English asks the question, and a native speaker wouldn't say it.
Write a short dialogue: someone asks how old your younger brother or sister is. Answer and add how many years younger than you they are.
— How old is your sister? — She's sixteen, so she's nine years younger than me.
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