Maturity plays a big role.
My girlfriend & I started swinging at the ripe age of age 21, introduced to the swing lifestyle by a late 40's couple while enjoying a nudist resort.
In the beginning we found ourselves meeting mainly older couples, aged 40-50. Couples sought us out as part of their fantasy. The women wanted to experience a younger guy, sometimes like screwing their son's friends. My girlfriend was uneasy with this at first, but soon found that experienced men lasted longer than guys her age!
Years later, we've found ourselves reliving our younger days, now 35, we enjoy to meet mid 20 couples and still enjoy the mid 40 couples.
It all comes down to maturity levels! We agree that age matters 3x in ones life - 18 to drive, 21 to drink, and 65 to retire. The rest is irrelevant!