German gives your age with sein (to be), as English does: ich bin zwanzig. You can add Jahre alt - ich bin zwanzig Jahre alt - but never Jahre on its own. The question is Wie alt bist du? or, formally, Wie alt sind Sie?
Examples
- Ich bin fünfundzwanzig Jahre alt.
- I'm twenty-five years old.
- Wie alt ist deine Schwester?
- How old is your sister?
- Mein Opa ist achtzig.
- My grandad is eighty.
The mistake to avoid
"Ich habe zwanzig Jahre" copies French or Spanish - German uses sein.
Where this fits in
Family & people, German A1: Talk about your family, say what people are like and how old they are, and introduce your friends.