I know I'm going to sound like some old codger here but I think a part of this problem is economics. Today, it takes both mom and dad working full time and taking any overtime they can get just to provide for a reasonably comfortable middle class lifestyle. Many kids get very little in the way of parenting and their only role models are other kids.
Not saying this is the only factor but I do think economic pressure plays a big role.
I'm not even old and I agree with you here somewhat. I work as a residential property manager and I see this everyday albeit in NZ not the USA.
I do have to disagree with the both parents working in the middle class though - I find that it is more common with the lower socioeconomic groups into this scenario:
Parent[s] on the dole (govt. benefit) and instead of feeding everyone and taking care of the family, they go do their own thing and leave the family to their own. I find countless bottles of piss, 50inc LCD tvs, ps3s etc - they are just plain selfish and lack any responsibility.
What else are their kids supposed to do - who tells them right and wrong?
Going on to chomps' one, I find that with the "richer" kids, its a sense of entitlement. Their parents give them everything they want, so who is to say they cant have everything else - gross generalisation but you get the point.
Both these here are two extremes, but with the former and seeing it first hand, its sad to say but its pretty common.