My parents were abusive (to each other and us). I wanted to run away multiple times--I was probably 9 the first time. I made it around the block a few times, but for whatever reason, I always came back.
For us, leaving was presented as a very valid solution to problems. If my parents were especially mad at each other, one of them would leave (and come back eventually). If we were misbehaving, we were told (among other things) that we could just move out if we didn't like the rules (starting around age 7 or 8 for me). I'm sure that (and the abuse) had much more to do with my desire to run away than any developmental stage I was in.