i think my parents just trusted me. cause i never had a curfew either. well, as long as i was at home/in my room before my father went to bed (he was a night owl up until 5 years ago, and when i was growing up, he was awake until at least 2am), and as long as they knew where i was, they were fine. then again, i never really did anything. all of my friends lived within a 5 mile radius of the house. and my best friend lived across the street. i remember many nights when i would be sitting in her yard and hear my mother yelling for me to come home.
though. my dad cared more. mom trusted me, and would let me make decisions. dad was more of the "where are you going? who are you going with? what are you doing? how long will it take? who else will be there?" kind of dad. but as long as they knew where i was, and i was in the house by the time dad went to bed, they trusted me.
HOWEVER.
when i have kids, they'll have a bedtime. 7:30 when they're small, 8:30/9 when they're bigger. up until they turn thirteen, and then we can talk. i think thirteen is an appropriate age to discuss bedtimes.