I think it depends on how you date. Sure there are probably young people out there who date lots of people and claim poly, but that doesn't mean some of them aren't. When I was "dating" I never had more than one boyfriend at a time. I didn't date other people when I was with one guy. If I was involved with someone else, it was with permission, a one time thing, and our relationship was considered "open"; not poly.
I don't really consider myself "young" anymore (I'm sure some of you do). I'm 25, married two years. I've been with my husband for five years but that certainly doesn't mean this stuff never comes up. Once we started looking at this type of lifestyle we realized this is one reason why you don't fit the "normal" perimeters of a relationship. We've both realized that there have been times in the past where we have been with one person and loved another, but felt ashamed for feeling that way.
So if young people are dating multiple people in a kind of casual manner and calling themselves poly, they're probably missing the point. Greenearthal is right, they're just using the term because it seems to fit their behavior.
So...that's my thoughts on it.