Then they shouldn't want to work at Google and Facebook. No one serious there works 40 hours per week.
Google New York is pretty 9-to-5 these days, except for people who are gunning for promotion... and it's pretty hard to build up the rolodex (due to the "2-up rule", which is that promotion from level N to N+1 will be decided by a panel of N+2's) to gun for promo every year. So most people work 9-to-5 (actually, more like 10-6:30, because of the dinner) except before a launch if they believe it will lead to promo.
MTV may be different, and I know nothing about Facebook, but I'd imagine that they'd be down to the typical ~50 hour weeks of stable startups. People expect founder status if they're going to be committing 60+ on a regular basis (i.e. not just a one-time push).
Make no mistake, people like the ones you describe -- listless ones who clock 40 hours while looking at the clock and describing every major shipping push as a "death march" -- are the ones you want to avoid like the plague.
Sure, but the problem with the clock-watcher isn't that he's only there for 8 hours. It's that if he had a better attitude, he'd be working instead of watching the clock. The fact that he leaves at 5:01 isn't the problem; it's that he's been mentally "checked out" since 3:30. Anyway, I don't know that that's an age problem. There are definitely 22-year-olds as well with bad work ethics and huge senses of entitlement.
It's also perfectly possible for a person to put forward 40 solid hours and accomplish an incredible amount.
An alternative and far more successful approach is to hire young guns who are intrinsically motivated.
You want a mix: young people who will support decisions they didn't make and put their heads down, and older people with experience who will push back, mentor the young, and try to keep them from burning out. The problem with the "young guns" is that they usually don't know how to pace themselves and that they won't push back against a bad idea. (No matter how awesome you are, some of your ideas suck.) Also, you have to question this "intrinsic" motivation. Is it because they have expectations of the job (or their career) that you won't be able to fulfill? That will become a problem later on, and it will blindside you when it does.
You just don't want to hire people who want to (a) be highly paid and work at the best companies in the world but (b) work only 40 hours a week on projects that are so unambitious that they are guaranteed to ship with no tricky bits.
What about category (c), which I think describes most good software developers: people who want to work hard on ambitious projects, but with high degrees of autonomy and without having to suffer unsustainable hours because of unreasonable deadlines?
You seem to have this black-and-white view where people either have an unconditional work ethic (i.e. either have never failed or haven't learned from it) or are completely listless and used-up. I don't think that accurately describes reality.