Polyamory is different than polygamy. At the age of 45, I entered into a polygamous marriage. We were together for more than 25 years until my husband died in 2018. Polygamy in Islam (at least in our case) is different from what I've read about polyamory. Although I loved my (future) husband very much, after much trepidation, a lot of reading up on the subject both from Islamic and secular sources and changing my mind several times, I finally married him. It was beneficial to me for many reasons. Initially I was so worried about what my family and friends would think but it became a non issue since everyone loved him and loved us as a couple.
People think of polygamy as a man "eating his cake and having it too" but in Islam it's not that simple. Such husbands have to please and support both their wives and treat them each fairly. Try doing that with menopausal women.
Ironically I was a jealous person before and during part of the time we were married. Didn't help that my husband was gorgeous and had lots of sometimes brazen admirers. Islam allowed polygamy (did not invent or encourage it) for two main reasons. During the holy wars, many women and children lost their husbands/fathers. The men who were left took on more wives in order to support them and their children. Secondly, It is said that God ordered: "Choose 4 and divorce the rest". That was because back in biblical times, men had several wives and concubines. And trust me, my polygamous experience was nothing like what they show on Sister Wives. Another point about polygamy. The wives are given respect and are entitled to their husbands' properties unlike mistresses of a large percentage of married men in this country who have admitted to having affairs.
A couple of years ago, the first polyamory couple I read about was in their 20s and engaged. A news site did an article about the practice and featured them. She had another boyfriend and a girlfriend. He also had someone else but I can't remember if it was male or female. Both had the other partners with each other's consent. You mentioned polyamory participants as being in their 20s, 30s & 40s but lets not forget the "swingers" of our generation. Also Baby Boomer gay men participated in some hot orgies back in the day at The Baths and places like that. How do I know this? Some of them wound up being our patients in the STD clinic and told us all about the lifestyle.