I had no word for it, and suspected that it was wrong to have these feelings, but I realised I was poly in my mid-teens. If there had been these sorts of resources out there then, I would have very definitely self-identified as poly back then.
So no, I don't think that any age-group has the monopoly on poly, nor should it.
"Listen, or your tongue will make you deaf." - Native American Proverb