My rule of thumb is to ask: Is the age gap big enough that it creates (or amplifies) a "power differential"? THAT is the key phrase.
An example of no power differential:
When I (a guy) was 22, I dated a 34-year-old (also guy).
But it wasn't a problem: technically, I was a college student-- but I was just finishing up one or two final courses, and basically stepping into a nice technical career. And he was journalist in a specialized and quite boring field,...
But he "lived like a 22-year-old" if that means anything (So: question that never arose: "Dahling, usual plates tonight, or the good china?") and about the only parts of life that he was vastly more experienced with was Microsoft Excel and a receded hairline. (...both of which I thought were cute, actually.) He had his own apartment, whereas I had to have roommates for a few more months,... and he had a car, whereas I was a little short of being able to afford one, and I just lived near campus-- But otherwise, no big deal. Aside from car and apartment of his own, we were just two dorks, so we were very much equals as far as I was concerned.
BUT... Conversely:
If I were still (at the same age, or at 20!) just an aimless college student, flipping majors, having no idea what I would do in life, staggering my way through a full course load with no end in sight, jobless, or maybe waiting tables... then it would have been harder to work as equals.
And if there were *other* power imbalances-- like if he were sopping with money, posh house, world traveler, and/or if I were has eleventh boyfriend instead of... fourth?-- then that would be a different situation, with very pronounced "power differential"-- And a 12-year age difference would have locked that imbalance into place.
I'm not saying that that's *inherently* unhealthy in a relationship, but it does sort of make the relationship less flexible: in some ways, the younger person is pretty much just tagging along-- which is okay, if that suits both parties, or if both parties are adult enough to acknowledge that, and communicate about it.