And I can't help but think that it kind of make sense to me? Honestly if I'm still an employee by the time I'm 35/40, then I messed up somewhere.

Your memory certainly isn't going to improve with time, it can only get worst AFAIK, sames for the number of neurons, your brain elasticity, your capacity to learn new stuff. For instance I can honestly say that my dad learned less in the past 12 years than I learned in the past 12-24 months, even when he puts his mind to it just doesn't work that well anymore.

In most cases you're going to be out of touch with young people(whether customers or employees), I think it's +/- inevitable and it's going to happen to me too at some point, that's just the circle of life.

Now this may come off as really evil, but you're also more likely to get sick or hurt yourself. Do I want to have carpal tunnel syndrome or osteoarthritis from typing 8 hours a day for the last 40 years? Not really...

You've most likely got less stamina too.

Statistically speaking you're less likely to change your mind on any subject, even if you want to, the older you get the more your beliefs are fixed, and the harder it is to change in any area. Not saying it's impossible, just harder.

Is there any non-emotional reason that causes people to think it's actually a bad thing?

Of course I don't think older people are worth less than younger people as human beings. I just think in a lot of cases it might not make sense business-wise to hire them/keep them.