Ageism is a real problem, but in my opinion ageism is often cited by job seekers where the problem is something else entirely. In tech, there seems to be quite a bit of discrimination today against employees who have worked for many years at the same position, and that discrimination is typically aimed at large environments. Startups seem to be ok with hiring older engineers who bounced around and worked in startups in the past, but they usually won't give much consideration to a 50 year old who developer has spent the last 25 years at an insurance company or major bank.
Take two 50 year old engineers. The one who has had 10 jobs over the past 25 years is considered infinitely more employable than the one with 25 years at the same department of the same company.
This case isn't ageism, it's the expectation of career stagnation, yet many of those engineers with that 25 year tenure think it is. It's a leftover from the days when loyalty was tied to tenure, and loyalty was valued highly in the hiring criteria. Those days are over. Loyalty is nice, but loyalty that negatively impacts your career options (no learning or improving, inability to use marketable tech skills, etc) is foolish.