I worked with great 60-year old engineers, but I also worked with more than one 50 year old who had not learned much in his 25 year old career, in the sense that he was doing the same mistakes a noob would do (over and over again I assume), except he's churning code much slower, and without knowledge of modern tools (some of which are really useful and effective)

Knowledge and professional experience are not properly reflected by age. Naïveté and lower expectations however usually are, which may explaing agist hiring preferences.

I would be sceptical of hiring somebody that has been code monkeying for 20 years, as in still making the same mistakes they were making 20 years ago. It's a smell of somebody that doesn't warn to learn new things, and has no commitment to quality in any sense of the word.

On the other hand, somebody that has deeply learned their craft and is still actively learning makes a very attractive employee. Age and number of years on the job by themselves say nothing about ability.