I feel this might change when more of the current young generation of engineers are getting older since there just weren't as many tech workers 20 years ago, so it's still relatively rare to see 50 year old developers. Besides there are many places outside of Silicon Valley or the US were experienced engineers are very welcome.
I agree. My experience has been that ageist views (just like sexism, racism, homophobia, etc) are more often expressed by people of previous generations and that millennials are actually more flexible and open to the idea that they would work with someone older (or of another gender, different skin colour, etc).
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.