I think I have (too often) thought the Pro definition was synonymous with skill and experience, and a broad spread of background knowledge, not just about your gear and what it can and cannot do, and light, and composition and all that good stuff, along with any life experience/education/etc. that contributes to what you are shooting or creating.
So my question has evolved from "at what point is someone considered a pro?" to "How do I define someone as a skilled and experienced photographer? At which point would I even dream of thinking of myself as a skilled and experienced photographer who is over the tipping point in terms of how much I have learned versus how much remains to be learned?"
The answer to the last question, I suspect, is never.
I think a pro delivers paid pictures that meet or exceed expectations (whatever those expectations are, and whether they were stated or not) and approaches assignments from the perspective of needing to be paid in a professional businesslike manner.
The definition of a skilled and experienced pro is much, much broader.