I've been a senior developer at the same company for 13 years. I feel that most of the time, I am progressing in my knowledge and experience, so, in my mind, I am making progress. It just doesn't seem that way on my Linked In profile.
Deputy Developer? Elder Developer? Doyen of Development? Development nestor of company X? Director of Engineering at Sub-sub-sub-sub-department that happens to be just your team? Level 20 Wizard? Does it even matter if it sounds good on LinkedIn?
Age-ism, of course. You can't be a 50yo senior developer.
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'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.
What kind of job would you be aiming for?