Lol so did I. Ageism is a thing, and it's everywhere. At least when you're young, you don't have the excuse of already having been in the other age class. That being said, several of my older professors were entirely full of snobby shit. The older I get, the more I see how they were not trying to impart knowledge, but to gain some kind of status as "hard-ass" old men with the younger generation.
Youth are always writing off the oldies - I did it, and now that I am old, I see it happening to me - and that is ok - we need that passion to shake things up, even if they end-up eerily similar to the way things were done before...
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.
None of it - that's the point. They are self-deprecatingly pointing out how naïve and judgemental their younger-self was.