Due to the fact that I'm 21, I'll throw in on this one.
In my personal experience, it varies from person to person. I know some people who have been in several long term commited poly relationships. I know 10x more people who claim to have been, but they really are just sleeping around without the title of 'slut'
I am a relationship person. The definition of relationship can be whatever I and my partner wish it to be. But they are relationships nonetheless. I stick with people. I do not sleep around, unless it's a night of fun with a partner and someone we find attractive.
I have a serious partner and a not-so-serious partner. This doesnt mean that I am just sleeping with the second mentioned, it just so happens to be that we tried many dynamics and this one works best for us. I am very very close with both of them.
There are other factors that help define my relationships. I have a 3 year old son. I am in school full time. I have almost NO free time and so building my relationships takes much longer than others.
I think it's all relative. Likely the reason you see so much frivilous behavior in younger folk is because they are young, and young adults, regardless of being poly, mono, straight, gay, smart, dumb, whatever, are just not really ready to be in the type of relationships that most of you are in.
It's all a learning proccess, I guess.