Okay, so, I saw this on Fetlife, and while I fully plan on posting about this there, I want to do it here first.
I get where the question is coming from - the labels dom/me carry a weight to them. Doms are the ones entrusted to keep a Sub safe while we're vulnerable. To not take advantage of us while we are bound/in subspace/etc. For a newbie to label themselves a dom might seem laughable for those of us that have been practicing BDSM for months or years.
The thing is though: we wouldn't feel the same way about a newbie calling themselves a Sub. (And we're just belittling someone for being new to something)
I was 20 years old the first time I attended a kink event. So I couldn't even legally drink alcohol - I was just a few months shy. I went alone, and the turn-out was more than 200 people. To say that I met a lot of people, and therefore introduced myself a fair amount of times, is an understatement of grand proportion. Each time I said I was a Sub, and never once was that questioned or scrutinized. Never once was that met with derision.
D/s dynamics are not one-sided. Subs are trusting doms with their well-being, but doms are trusting subs with theirs, too. Doms have to trust us to communicate our limits effectively, to set boundaries clearly - because no good Dom wants to cause genuine, malevolent harm to their Sub - and they have to trust us not to cross their boundaries, too. They don't want to find out after a scene that you needed to use a safeword, yet chose to remain silent. They don't want to find that their Sub wasn't enjoying themselves, but were too shy to speak up. If a Dom says that they don't like it when a Sub talks back, then that Sub better be obedient or choose not to sub for that Dom.
The labels Sub/Dom are to communicate the roles we prefer to take on in the bedroom. There are no standards or requirements that need to be met to adopt them. Some might need to explore to figure out what they like, but some, like me, just know. A newbie Dom is an inexperienced Dom, but a Dom all the same.
The idea that they'd need to 'work up to it' is a unnecessary double standard that could only alienate a Dom that's trying to educate themselves. As if you need experience to be a 'real' Dom.