A student once asked me, “when can I call myself a swordsman?” I think he was expecting my answer to be a certain number of years spent training, but I said “when swordsmanship is your primary means for analysing and solving problems.” This was not what he was expecting. It is why I think of myself as a swordsman, but it's not strictly true, either. You can call yourself anything you want, whenever you want.
The bigger question is why would you want to?
I never set out to ‘be a swordsman'. I just spent so much time practicing and teaching swordsmanship that I just ended up being as much a swordsman as I am anything else. I never aspired to be a writer, either. After I had five books out, one of my friends chided me about my lack of typing skills, and used the phrase “you're a writer” to justify his criticism, and so I started using “writer” as a convenient, true but not accurate, shorthand answer for the question “what do you do?” when I don't want to have to explain the whole swords-books-courses-school thing.
I did once aspire to be a cabinet-maker. And I became one: it was my full-time employment from 1996-2001. But I wasn't actually very good at it (by the standards set by my colleagues. I was competent; they were astonishing), and wasn't very happy doing it. Being it. But because it was “who I was”, it was much harder to change than “what I do”.
I do understand when somebody asks me what they should do to “become a writer”. My best advice is this: don't. Don't try to “become a writer”. Write. Publish. Be read. And if you like it, keep doing it. What you are is a human being. What you do is “human stuff”. It's a fault in our language that we ascribe identity to actions. He is a shoe-mender. She is a police officer. Better to say “he mends shoes”, and “she polices”. I would extend this into most areas of life. “I am afraid” is a very different statement to “I feel fear”. “I am hungry” is different to “I am experiencing hunger”. One format identifies you with the feeling, the other does not. It's worthwhile being very careful about what you choose to identify as; so much so that it is probably better to avoid identifying as anything by default.
If you want to “be a writer” more than you want to actually write, then “being a writer” will probably make you miserable. I suggest doing the things that you love, that feed you, that you believe are important, and leave the definitions of what that makes you out of it completely. Let other people define you (you can't stop them), and embrace those definitions if they please you. But for goodness' sake, don't aspire to “be” a thing. Just get on and do the things that you want to do.
It's like being a good person. Very, very hard to be good consistently. But behaving like a good person? Much easier. And from the outside, a well-behaved villain is indistinguishable from a good person.