It is completely arbitrary. At our company the entry level developer is "Senior <language> Developer". It is literally the lowest developer title you can have.

It's all over the place. A college dropout who starts a start-up as CTO then fails within the year will apply as a senior. But, it's one of those things where I don't judge a book by its cover in either direction. After talking to the person in the right context there's a certain confidence and deep level of understanding that I expect from senior engineers and it never reflects their years of experience.

Yep, I agree with all of that. A lack of a cut-and-dried definition in terms of years or other objective measurements makes it hard to compile data, but that is exactly how I think of a senior developer.

agreed -- it varies significantly based on company culture, size