I understand the general concept of "als" = "when in the past" (pardon the oversimplification), however all the examples I can find always refer to very specific moments in time, and to events that happen only once, for example (silly):
Als er den Preis bekam, sagte er "Danke".
This is obvious, since the event "als" refers to is quite clear and only happened once.
However I'm not sure about things like
When I was 20, I had a fast car.
I have a (possibly totally wrong) feeling that it should be
Wenn ich 20 Jahre alt war, hatte ich ein schnelles Auto.
But I'm not quite sure. Should "als" be used in that case too? Or is there yet another (unknown to me) alternative?