It's far easier, I believe, to get involved with things like that when you're younger - not only because you're more likely to have the physical attractiveness factor working for you, but I remember just being far less concerned about everything in general when I was that age (early 20s). I think somebody who is younger is less likely to worry about the possible consequences or implications of group sex, and just do it because it sounds like fun.
On the other hand, with life experience often comes a better sense of proportion and perspective, and some people no doubt begin shedding their sexual hangups as they get older and want to experience something like that before it's too late. Really, everybody's different, I guess...some people 'loosen up' as they get older, and others go the other way.
My one-time threesome happened five or six years ago, when I was in my mid-40s, and involved me with a couple. I guess like most real-life experiences, it didn't exactly live up to the fantasy in all aspects, but it was fun.
It really is one of those stereotypes that spring from truth, I think, that men - not every man, obviously, but men generally - tend to take sex far less 'seriously' than most women do. While I'm sure that there are women who are every bit as sexually uninhibited as any man, right here on this board (and by all means, contact me if you're ever in the Phoenix area
), I don't think that's the norm. I remember hitting puberty at 12 or so, and wondering why people didn't just go around having sex all the time; I guess I've learned and matured somewhat in the intervening 38 years...oh, bullshit, no I haven't...who am I kidding?