I'm a little confused, because I wasn't aware that there was career progression beyond Senior Developer. I mean, you can go and lead a team or something if you want I guess (in fact, I'm doing that at the moment), but most older developers I know have been and done that, and settled back in highly-paid, highly-respected, and much easier individual contributor / architectural roles. Looks like a good life to me!
The best developer I've ever had the pleasure to work with was a 50-year old senior developer. He cut his teeth doing a lot of C/C++ stuff back in the day, but was also (pretty successfully) leading the company's adoption of Angular. If you have a sharp mind, and you don't get stuck in your ways, then people will be begging for you to be their 50-year old senior developer.
I know more than a few places that have Principal Developer positions. This is basically for senior devs who have tons of domain knowledge that companies don't want to lose.
Um, I know lots of 50 year-old (and older) senior developers.
I'm not ignoring the fact that ageism is a real thing (it most definitely is), and it is more difficult for many older programmers to "keep up", but that doesn't mean that no one is doing it.
The best developer I've ever had the pleasure to work with was a 50-year old senior developer. He cut his teeth doing a lot of C/C++ stuff back in the day, but was also (pretty successfully) leading the company's adoption of Angular. If you have a sharp mind, and you don't get stuck in your ways, then people will be begging for you to be their 50-year old senior developer.