(I'm not the parent commenter), but I believe a defining quality of a senior developer is someone who I can give any problem/project to in the business domain and they return with a solution in the technical domain.

> What's your definition of senior?

I have already made the mistake you are about to make.

You've already invested a huge amount of time in learning something which became obsolete (like jQuery), then you got over it instead of giving up in frustration, and spent a huge amount of time learning something else (like Angular or React), and you've finally come to grips with the fact that it too will eventually become obsolete, and you'll be fine moving on to the next thing (like Svelte).

If you only know one system and think you're going to be able use it forever, then you're still a junior developer. If you're afraid to move on and learn something new, then you shouldn't be a developer at all.

Someone who's reached their last year of high school. Seriously though, senior just means you're older and have more years of experience. Make sure you help your kouhai when they're in trouble.