"I want to stress the importance of being young and 
    technical," Facebook's CEO (now 28) told a Y Combinator 
    Startup event at Stanford University in 2007. "Young people 
    are just smarter. Why are most chess masters under 30? I 
    don't know. Young people just have simpler lives. We may not 
    own a car. We may not have family. Simplicity in life allows 
    you to focus on what's important."

I'm on the younger end of the spectrum (31), but I just don't see how any CEO thinks you can say something like this and not open yourself up to age discrimination lawsuits. If that's his opinion as CEO, you can be sure that that attitude impacts policy either directly or indirectly.

Is this actually true (or is it more the experience of a few people generalized to everyone)? How do you avoid this?

Well there is one quite obvious way to avoid this, but it has a serious downside.

I did not take that route, thus I am now over 50 :) My advice is to live on way less than your means while you are younger, and then you can have an air of nonchalance as you age, giving the impression of great wisdom ( hopefully somewhat backed up by reality)

Maybe move into academia, finance or found your own startup.