I voted post-marriage only because hey, I'm old-fashioned. I think sex ought to be respected and not flung around like some casual activity.
But from a non-religious standpoint, I might have said college, because it is a much larger part of American culture to have sex throughout college than in high school. I think high school is a little premature-I mean, I don't think many relationships can really develop to a point where sex is appropriate to supplement their other intimate activities.
I don't think age is the real issue when it comes to having sex. I think sex done without proper protection or understanding of the consequences is inexcusable. I also think early sexual promiscuity leads to a lack of appreciation for sex between those in a pure, intimate relationship.
I'm probably not a credible source though, I'm a virgin. Which shouldn't be such a big deal in our culture! Besides, I'm uncomfortably gay, like I'm going to run out and... well, have sex.