IME it's not pure 'ageism', but salary and workplace dynamics. Having had the privilege of working with many older devs, who played a big role in shaping my career...
After the first 2 decades your worth as an individual contributor (or at most, team lead) stagnates. You're probably at the top of the pay grade already, and the older you get the more you'll have to work with more junior people, as 'equals' despite having substantially more experience. It's very hard to give you any sort of career progression. You may be a lot more productive, but your 'multiplier effect' is small compared to a manager, or someone who deals with a lot more stakeholder complexity.
It's even more pronounced in current times where a 25 year old makes 100K (a lot in the UK). They're very unlikely to double their salary over the next 10 years in similar positions. Whereas most people start at 25K.. work their way up to 100K... exec level etc is 200K.
Successful 'pure' programmers who get jobs usually have an infra flavour of the month skillset, or make it clear that they're happy to get paid the same as someone with say 10 years of experience.
Most of them though go into strategy or become contractors.