。
How old are you?
Literal
What years-old [copula-です-question-か]?
何歳 ('what age') uses the same 何 + counter pattern as 何時 ('what time'), 何人 ('how many people'). A more polite alternative is おいくつですか, where いくつ is the polite indeterminate-quantity word and お adds politeness — preferred when speaking to elders or in formal contexts.