"Did you have a car when you were twenty?"
Translation:Hadde du bil da du var tjue?
No (it can be though). Many norwegian sentences drop the indefinite article, but you still need them in the english translations.
Jeg kjører bil = I am driving a car.
Hadde du tatovering når du du var yngre? = Did you have a tattoo when you were younger?
I think we drop it when we are talking about a thing more generally, for instance, Hadde du bil da du var tjue? doesn't really ask if you had one car.
"Da" is used when describing single occurrences; things that only happened once.
"Når" is used when describing recurring events (think "every time when...").
Since you were only 20 once (even if it was over a period of time), "da" is the correct option.