"Did you have a car when you were twenty?"
Translation:Hadde du bil da du var tjue?
I think you're talking about the case when things are not countable. The article is also omitted in English in this scenario. "Jeg trenger vann" / "I need water".
There is an alternative explanation also. "Mysteriet Om Nils: Part 2" claims that the indefinite article can be dropped when there is usually only one of a thing. It gives examples including "Han bygger hus", "Per skriver brev" and "Jeg kjøpte ny bil".
The last one is interesting, because it's also given in a completely different coursebook, "Stein På Stein", but I have had a native speaker tell me he'd never say this, and would always include the article, although he would say "Jeg kjørte bil". So, maybe the Nils rule is idiom-specific, and there is some variation depending on dialect / generation / whatever...
"Hadde du bil da dere var tjue?" should be accepted. I know it seems contradictory at first but the person we're talking about might have a twin brother (or a friend who's the same age as them) who has never had a car, which is why he's not included in the question. In that case the sentence makes perfect sense.