To me, if you write you are a writer. If you get paid for it, even once, even a little bit, then you are a professional writer.
That said, I didn't start calling myself a writer until I'd finished two novels. Then, it seemed pretty obvious to me--even though I hadn't yet gotten a dime for my work.
I think validation as a writer is highly subjective. What does it for me might not for someone else, and vice versa.
I think it's the word "validation" that's a problem. All I know is this. Anyone, even someone with zero talent, zero skill, etc., can sit down and ruin paper by getting ink all over it. And from reading slush piles, this seems to be the case more often than not.
I suppose it is a subjective decision, but I do not want to be something that anyone can be, no matter how bad they are at it. Quality has to come into play somewhere, doesn't it? Unless you want to add the qualifier "bad writer."
And are you really a professional writer because you sold one piece to Podunk Weekly three years ago, and haven't sold anything since? The root word of "professional" is "Profession." Can you really say your profession is that of writer when all you've done is sell one small thing? For me, that's like calling yourself a doctor because someone pays you for the band-aid you put on a cut.
Subjective? Yes, of course. But it's been my experience that those who succeed are most often those who do not claim victory before they're really achieved anything, and while real effort is always noble, ruining paper by getting ink all over it is not much of an achievement. I've certainly ruined my share, and I never once thought, well, that makes me a writer. Those who succeed are usually those who say I'm
trying to become something, not those who say they
are something just because they try to be.
Maybe it is a subjective, personal standards deal, front to back, but darned if I'd have the guts to tell anyone I was a writer just because I sat down and wrote something. This makes just about all the literate world, and most of the children out there, writers.