To me, Senior isn't a measure of time and experience (it definitely isn't at my org anyway). Some people are more than happy to be an intermediate forever. To me, a good senior should have a better understanding of the complexity, risk and big picture for a change, and have the capability to design better systems in more elegant ways. Sure, some people gain those abilities with time, others just innately have them, and others never will.

Focus and steady discipline. (Im not a swe im in security, but I do code a bit.)