I'm sorry that triggered you.
But I wasn't suggesting she give up on polyamory entirely. Just that since she felt it was at a dead point and they sound like "messy people" that she consider not being involved with them any more since they are not meeting her needs. Date people who are more compatible.
Some older people have an annoying habit of treating ALL 18 year olds like kids just because SOME 18 year olds have not reached their emotional maturity yet. Used to drive me nuts at that age! So I'm sorry you have experienced that to the place where it triggers you. Not everyone thinks "18 is just a kid."
I think if an 18 year old is ready to go there and assume the mantle of adulthood -- go! Do what needs doing in your life to leave the family of origin and start to fly on your own!
And part of knowing one's own adult business is knowing when to pull the plug on something that just isn't a runner and doesn't feed what needs feeding.