Should I use the preterite or imperfecto in this sentence?

Should I use the preterite or imperfecto in this sentence?
 

Ángela y Roberto se llevaban muy bien. Pasaron/Pasaban una juventud muy feliz juntos.

The first sentence conveys that they were getting along well and that that hasn't changed. It wouldn't sound right to say that they were happy (start and end)together when they were young in the next sentence.

It depends on what you are trying to say. What follows? (without a context "una juventud" suggests pasaron, "su juventud" suggests pasaban and "la juventud" any of them according to what you're trying to say)