French gives your age with avoir (to have), not être: you HAVE twenty years. The word ans (years) is never left out, and the question is Quel âge as-tu ? or, more formally, Quel âge avez-vous ?
Examples
- J'ai vingt-cinq ans.
- I'm twenty-five.
- Quel âge a ta sœur ?
- How old is your sister?
- Mon grand-père a quatre-vingts ans.
- My grandfather is eighty.
The mistake to avoid
"Je suis vingt ans" is wrong, and "j'ai vingt" on its own sounds unfinished - say j'ai vingt ans.
Where this fits in
Family & people, French A1: Talk about your family, say what people are like and how old they are, and introduce your friends.