though my wife and i are only discussing the possibilities of swinging at the present time, i have my own opinion here based on my experiences...
Im in my mid-30's...my wife and i have been together for 7 years now (i think...*LOL*). If we were to start playing with other couples, i'd look at it like "this is where we've progressed to as a couple."...i think most swingers are the same way, and i think this is really what swinging is all about.
now, when i was younger (late teens and early 20's) i had several experiences with same bed sex...but at the time we just thought it was part of being young and wild. in fact, i dont remember being at all aroused by what the other couple was doing, i was just focused on the girl i was with. The real reason we were all on the same bed to begin with was that its often hard to find a place to have sex when you still live with mom and dad! *LOL*
In fact, my very first sexual experience was a MMF threesome 3 weeks before my 15th birthday, when a girl who was much older decided to "educate" my best friend and i.

Again, i was just trying to concentrate on what i was doing.
So my point is that people as young as that are really just coming from a different perspective...i suppose you could call it "swinging", but i'd just call it experimentation...and yes, i definetely think experimintation is a good thing!