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.