To start with, "Senior" and related titles basically mean a person who doesn't need much hand-holding. This might be related to time spent, but often you find there's a lot of variation. People I consider senior software engineers have a can-do-anything vibe about them. They have a wide range of skills in the field, and have that T-shaped experience profile: there's some area they seem to know everything about, and t…
This is my definition. Can I assign the PROBLEM, "important customer ABC has some kind of weird bug, talk to Nav in Tech Support" and know that problem is now off my plate. They'll usually solve it or come back with a hot fix recommendation. Worse case, they come back with a "we can't figure it out, we need more eyes on this".
The important part is this: the problem is now something I don't need to worry about.
If I need to follow up, they're not a senior developer. They're a junior with skills.