Think it's actually a paygrade thing at some companies. Get a talented engineer (that yes, probably had open source projects already as a teen) and they kinda want more pay than an average engineer. Now they're a senior engineer.

Of course whether said engineer is actually worth said salary isn't immediately obvious and oftentimes you end up with young seniors that are better at selling themselves than they are at engineering.

I also know at least one company I worked for developed clear outlines of what your skills (both soft and... Hard?) and ownership of codebase should be for each level. At the time I had just over 3 YoE (2.5 from previous company and was essentially one biannual review away from becoming a senior, but threw it away for a more exciting job and since then I've not been at a company that differentes levels at all. So I don't know if I'll ever be a senior now lol