Translation:How old is he?
Character by character, going by the tips, this reads "he/him + how many/a few/some + years old", which made me curious about how the second character works: can it be both a statement (a few/some) AND a question (how many)? How to tell when it is which, and are there other usage notes for it?
as someone who's minoring in chinese at university, and so now only uses duolingo as supplementary work, i've come to realise just how poor a lot of these are - you'd more commonly use 他/她/你多大, as 他几岁 is more appropriate when asking a small child (less than 10 years old)
You can say it, but normally you would just say "how old is he" if you were just asking how old someone was. However your sentence could be used and even though it is an unusual way to say it, it would be OK.
If you were asking someone to read somebody else's age from a field on a form or screen then you might ask "what's his age" in preference to "how old is he" because you are making a reference to a specific field on the form for "age".
That's only used in a certain context. You'd say something like that if you heard someone's age and were surprised by it. Like you see someone who looks 30, and someone tells you he's actually 50, and you would say in surprise, "He's HOW old???" But you wouldn't say that as a neutral question.