I work out in Asia and over here a big reason for ageism in tech is a misunderstanding of development talent, there is a belief that coders are coders and that's all there is to it, young coders tend to be willing to work longer hours and basically work until they burn out so management sees them as the superior option since salaries and benefits are lower.

edit: Since I'm getting down voted for whatever reason here's an article that is along similar lines https://www.bloomberg.com/news/features/2018-05-02/china-s-t...