Well, someone posted the Canadian thing, so I'm done on that.
With personal say, I always thought (in the case of heterosexual marriage, do to reproduction and all that) that someone should be in a position where they can care for an "accident" in the "worst case" scenario. I don't know, maybe they can't have an abortion or put up for adoption for some odd reason, but yes. If they can care for a child. Not saying that all sex should be reproductive, just saying, you can go in armed with enough protection to stand against the Russian tanks circa 1940-45, and there's still always that teensy percent risk, and I always bid on the side of caution, if only because not doing so in that case won't kill you.
And of course, I'd prefer it if the two people were at least in a dedicated relationship, but now I'm just day dreaming. Basically, I want the people to have plans if things might not turn out hunky dory. Is that too demanding?
And on the stance of gay sex, I'd prefer if they were in a dedicated relationship, with them not having the severe worry about pregnancy.
So, I'd say 18, but if you happen to be the manager of a MacDonald's at 15, and you and your girlfriend are mature, dedicated people, and you're ready for the scenario (whether you're expecting it or not, that part I don't care much for), I'll live with it.
Some people may think I'm sounding mean, but I always want to think of the children. On the case of abortion and adoption, that doesn't necessarily sit too wrong with me, but I've always found taking that responsibility more admirable, is all.
Oh yes, STDs. Well, in general case, this would come from being active all over, right? Or maybe being active with someone who was. In the former case, I'd never advise that, because that would contradict my idea of a dedicated relationship. Unless it's in your past, of course, then I simply advise caution in the standard ways, but if you're active with everyone, I can't condone with you, trouble seeks what trouble gets, and I don't consider this act a past time. Protect yourself at least, but again, always double check. Clinics and all that, doctors are very understanding people. I would hope, in most areas.
In the latter case, well, some people who've fooled around a lot can turn and say they're done, or something, and be a "pure soul", or whatever you might define them as. I suggest that (you not being this person, but being in a relationship with them) also ask them to go to Dr. Clinic for a check up, but if you can be 100% sure without, go ahead. But if you can't do the first, and be sure of the second, be cautious, in the standard speal.
Again, I may feel like I'm being too harsh, after all, what do I know? 15-year-old virgin, but I know too many younger than me that aren't even thinking about this, and I'd at least like for them to do so.
In conclusion (tl;dr) - I say if you can support an "accident", in case of such thing occurring, and you having no choice but to care for it (if it's such a bad thing to care for it, it's best to assume people don't want to, I suppose), you're in a dedicated relationship, and you and your partner are free/prepared for any STDs that either may have contacted, it's fine by me. I'd say I'd prefer people be older than 18, just so the majority of hormones calm down, but if you can manage maturity at a younger age, I suppose I'll live.
But keep in mind, everyone, I'm picky about this, if only because people who do so too young are somehow off-putting to me. I can't explain why, not a religious thing or prejudiced, merely something about the attitude that the majority of these people portray, and me seeing/thinking "no good reason" really.
