* They tend to get stuck on old technologies that nobody uses any more. I don't really care about your Perl skills, sorry.
* They expect higher salaries.
If you keep up to date with your skills (Rust, Typescript, Nix, etc.) and aren't precious about your salary increasing forever then I don't think there's an issue. I've worked with plenty of very skilled ICs into their 60s.
It continues to boggle my mind how many resumes out there are people who have spent the last 20+ years working on the same set of problems, using the same tech. The reason a lot of these people aren't getting jobs is not that they're old. It's that they've stagnated to a degree I would not have thought possible. Meanwhile there are people out there who are staying relevant *and* bringing extra wisdom to the table. Those people get hired.
There's nothing wrong with someone turning their brain off and working in that role for 20 years. But they shouldn't be surprised if their experience is completely irrelevant in the modern job market. If one wants to develop new experiences and their job won't provide that over a long period of time (say 5-10 years of employment, not every 6 months!) then it's time to consider the tradeoffs you're making.
I've hired for Java roles in the last handful of years and this was a very bimodal group.
Someone could be working on instant messaging or a/v conferencing for the past 30 years. Or anything in sales and advertising. Etc.
I'm only referring to hireability in new roles. If someone is staying in such a role that entire time, especially if their niche hasn't evolved such that it indicates refreshing tech skills from time to time, they shouldn't be surprised if the rest of the world has moved on from where they've stayed.