Well, there are two things going on here.
One, there's the issue of the pervasive economic insecurity facing Americans under 35 when compared with previous generations. Nobody's ever been in control of their political destiny, which is what the article's talking about, but that did not necessarily affect you on more than an intellectual level if you knew you'd be fed and dry. Economic uncertainty breeds a very personal kind of fear and paranoia, and it seems pretty natural to me that this would color one's larger worldview, especially when that looming spectre of poverty encourages you to empathize with the great mass of poor people worldwide who are ceaselessly fucked by international conflict, no matter who wins.
The other issue is that people under the age of 35 today don't view US foreign policy as an aberration within a generally right-thinking political culture. With Vietnam, the young could say it was a terrible idea and protest it but at the same time look to the Kennedy and Johnson administrations and see a lot they agreed with, otherwise. It was a simple thing to decide that the government was trying to do its best but the existential threat of a nuclear USSR was provoking it to do some dumb, dumb things.
Today, the difference is the young no longer see American foreign policy as independent from domestic/fiscal policy. After Iraq, there is just no way any of us are going to be able to separate war from profiteering, even in a hypothetical situation where a use of military force would be reasonable and just (I would submit Mali as the most recent example). This really cannot be understated; young people today attribute all sorts of malfeasance and bad agency to their government, but very much unlike the hippies or even the Reaganites they feel they can't do anything about it, and this is unspeakably terrifying.