Somehow, the question of "what age is it acceptable for vampires to date" has led to a discussion about whether or not Donald Trump is evil.
I guess the question of whether or not he's a vampire has already been addressed.
Being serious now, I think the primary factor in this dialogue has less to do with the amount of centuries on the vampiric clock and more the mental age and absorbed experience of the night-dweller in question. Are they actually a deific fixture of time that was old when Man was young? Are they merely centuries old, but capped around the last human threshold of mental development before going into something less understandable? Did they get frozen into whatever frame of mind they were in, when they were turned, and can only experience events through that lens?
And, of course, what age do they present as, and to whom are they presenting to? A fifty-year old appearing vampire striking up a relationship with a consenting young woman is, in a way, more honest than a younger-looking one purely for the visual gauge an uninitiated party can make of where on life's path this individual lies. Of course, there's debate to be had about relationships like that even before adding blood-sucking predation into the mix.
The thrust of this conversation is actually explored in my favorite piece of vampire and romantic-horror fiction, and the train of thought goes pretty much the way of every person's opinion here simultaneously.
In "Let the Right One In," a novel adapted into a Swedish and American film each, vampiric element of the book, a (castrated boy named Elias) "girl named Eli" is a twelve-year old vampire who has lived for somewhere around two centuries. His assistant, a pedophile named Hakan, helps Eli by murdering people to obtain their blood as a means of having a "safe" way to sate his pedophilac urges since he can rationalize Eli as being much older than him and thus it is not wrong to desire someone who looks like a child. This notion is challenged by Eli's rise into a lesser form of depression (even happiness) upon encountering another child, at which point Eli begins to act like a child and is generally much more energized and dopey rather than depressed and hollow.
The pedophile eventually dies, by the way, and comes back as a monster trying to prey on Eli before being beaten into a twitching goo-pulp by a third party.
The protagonist, friend, and eventual life-partner to Eli is a thirteen-year old boy named Oskar. Oskar meets Eli as two kids and assumes he is the age presented for much of the book, even after the revelation of Eli being a vampire - but the thought haunts him. Is Eli actually an old man inside, laughing at him, looking to create a new helper after her old one dies?
Even Eli(as) doesn't really know, judging by the perspective chapter we get on his part. But when the reveal happens, Elias is adamant: "I'm twelve, but I've been twelve for a very long time." When under stress and isolated, Eli acts somewhat more mature, is coldly practical, and clearly more intelligent than any child could be with a mere twelve years of experience. But outside of those periods (and even within them, sometimes), Eli acts like a kid and engages emotionally with his surroundings as a child would. Oskar makes an observation that maybe the reason he can notice things that Eli (who can solve a Rubik's Cube in about six hours) can't is because Eli is "younger" than him and thus less developed.
All of this informs my perspective on the question of the thread, which is: "What age is the vampire presenting as, what age are they in reality (mentally), and where on the human age line is their potential partner?" From there I think it can be pretty easily puzzled out in most cases where squick and immorality arrive and romantic partnership departs.
