"What is your age in years?"
Translation:Wat is jouw leeftijd in jaren?
It means how many whole years have you been alive for. It's not a sentence you'd normally say in English. You would just say "How old are you" or "What is your age". But you may see it on a form, especially if it is referring to children. You often express babies ages in months, or children's ages for example as fourteen and a half. They only want the whole number of years.