Some of them bought into the libertarian craze, so they consider older people useless, and anyone useless is garbage that can be tossed away; but it does not only come from that side: other ones consider the previous generations responsible for anything bad (and for a wide definition of 'bad') that happens to them, and those guilty people (indistinctly guilty) should be punished.
Are these extreme cases? Probably. I hope so. But I do not remember such a trend in previous generations.
To go back to the point about work, I have never witnessed or read the previous generations express any problem working with people older than them, on the contrary, they showed (some) respect and expected some knowledge to be gained from them. I also think that the age stratification in jobs is higher now than ever. The workplace is less and less varied over the years (I do not give a damn about skin colour, I am concerned about age and social origin), and tolerance (and simply knowledge) of people with a different social background goes down.
I think millennials are far more open when it comes to sexism, racism, homophobia, etc by far. However, when it comes to ageism not so much.