[slyn]

Roughly speaking:

If you have a hard time contributing to starting on projects or resolving problems, AND you have a hard time contributing to finishing projects or resolving problems, you’re a junior X (where x is programmer, engineer, sysadmin, etc).

If you can contribute to starting on a project/problem but have a hard time finishing it, OR you have a hard time starting on a project/problem but can contribute to finishing it, you’re an X.

If you can contribute to starting on a project/problem, AND you can contribute to finishing a project/problem, you’re a Senior X.

Not sure the current term of art to being able to start and finish projects / solve problems and your work regularly being good and reliable, maybe Architect or something like that.

[Alex63]

I like this heuristic! It captures several important aspects of "seniority" in a clever way.

A lot of the other comments are defining seniority in terms of years of experience or breadth of skills developed. Those are both important, but for me they don't capture how "well-rounded" you are. When I was a manager in consulting we used to distinguish between resources who had "5 years of experience" and resources who had "1 year of experience 5 times". Your years of experience need to be extending your capabilities, not just repetition of skills you have already mastered.

I think simple rules of thumb, like "5 years is a junior" or "5 years is a senior" aren't very helpful. When I'm hiring I may suggest a certain amount of experience as a requirement to indicate to candidates that I see the role as suitable for someone "entry-level" or someone further along in their career. But I'm not going to reject a candidate with good experience simply because they don't have the number of years experience I suggested. I'm going to look at what they've done.

One more thought on this topic. For me, years of experience can also be shorthand for variety of experience. Someone with 3 or even 5 years of experience is unlikely to have worked on a lot of big projects in different organizations. Someone with 15+ years of experience is more likely to have worked on a variety of big projects in different organizations. Some commenters here are probably going to tell me "5 years of experience at a start-up" is as good or better than 15 years of experience in a big corporate IT shop. Maybe. Depends on the role I'm trying to fill.