Yes, tigger, I do think their contracts are a factor, but not necessarily in the form of overtime pay as in your example (which is interesting and I never knew about before). The older workers are more likely to still have pensions (not offered to younger workers), they're paid more, have much longer vacations, and thus feel more indebted and loyal to the company.
The younger workers aren't as "bound" to the company since with no pension plan (you can just take your 401k with you!), only two weeks of vacation, and ever-shrinking benefits there's no reason to stay at a company more than a few years, and definitely no reason to do more than you're paid to do. I also sense that the younger workers can't spend evenings at work because of a cultural shift. Today, two salaries are needed (or at least desired) and women are encouraged to have careers, so most people have working spouses and are needed at home to help out, pick kids up from daycare, sports, school, etc. If one spouse is at work 60 hours a week, the other spouse has to do EVERYTHING with the household plus work a full-time job, which just doesn't work. Maybe someone here has a different opinion.