I think there is more going on here with this particular situation, being based in South Dakota does not help them keep employees, as they can run off to the oil fields and get a $7 per hour raise pretty easily. So this particular situation is probably a little more complex. Also, what portion of the kids were going to head to college within a few months anyways?
That being said, I do think there is a different work ethic among millennials than that of the older generations. Speaking from a work ethic standpoint, I have never felt that I identify with the majority of people my age, as I am kind of a workaholic. When I took my current job, and began being responsible for hiring and managing a crew of about 15-20 people, I had a very hard time with the young guys. The nature of my company means we hire pretty much all young kids. At first, I thought they were all super lazy and I did not respect them at all. After about a year, I decided I had to change my view on the subject, or I was going to go nuts. I decided they could not all be lazy, they just worked differently.
My employees don't get as muh satisfaction from hard work or even accomplishing something, as they do in being "a part" of something. My job has become selling the company to my employees, and convincing them that their hard work is somehow benefitting them through some way. With some, I have to act like a teacher and give them "valuable life lessons" and with others I have to give them titles and responsibility. It sounds cheesy, and probably applies to other generations as well, but it works.
So when I see other managers saying that millenials are lazy, I see more of a problem with the employer/manager than an entire group of employees.