I too got started in swinging young. Hubby is much older than I though -- by fifteen years. My first multi-partner experience was before we were married and I wasn't old enough to drink either
So, I'm thinking back to that time now and considering...
I wasn't a virgin at the time and I was far from chaste. It wasn't all that uncommon for me to have sex with more than one guy or girl in a 24 hours period. While I hadn't had a large number of parters (under 5) I did keep those partners into overlaps (wasn't monogamous). At any given time there were 3-4 guys available for sex before I was old enough to buy cigarettes.
Now, If I was having sex with two guys in the same day anyway, how far a step was it to go to an MFM. Too, if I was enjoying sex with
Jane on the side while also playing with her boyfriend
Dick at other times, how far a reach is it to enjoy
Dick and Jane (FMF) at the same time? And, since I was playing with
Dick and Jane -- why not invite my other BF
Tom to join in?
So in my case it was a natural progression from my promiscuity to the lifestyle. I liken that to my common argument that "if I'm gonna fool around by my sexual nature, I had better be married to a guy who likes women that fool around..."
IMHO - it's not about age as much as it is about the sexual nature of the people involved.