I would point out that those are older also tend to write off the younger. I think it's just perspective mismatch; If I can emulate another person's perspective in my head, I can anticipate their decisions (and reasoning), so I can decide if they are being reasonable.

However if I can't understand their perspective, I have a very hard time in understanding and judging their reasonableness (because I'm basing my judgement solely off of my own experiences and memories that are similar to their circumstances).

This lack of understanding translates to seeing a lack of credibility in them. "Maybe if they were more like me, they'd make more sense, be more reasonable". This type of thinking is common in most types of prejudice.

It's why young people write off older people: "They're too older to remember what it's like being my age, or to understand how things are now".

Why the opposite occurs: "They're still too young to understand how life works yet".

Why people of very different cultures tend to be prejudiced: "Their kind are ignorant of how the world works", and the opposite: "They've never been through what I've been through, they don't understand me or mine".

All of these statements evaluate down to: "If they were more like me, they would be reasonable". Which is of course true, if "they" were more like "you", their systems of reasoning and value be more similar to yours, and vice versa.

In computer science, it's particularly tempting for the yutes to write off the oldies, because technologies change so rapidly — I sometimes frighten the kids by mentioning that I got my first degree before the WWW was invented, and I'm far from retirement age.