I wouldn't say that I have enough investment in this discussion to call what I'm going to do "playing devils' advocate" but I would like to bring up issues about age differences in romantic relationships so that you (and here I address the OP) might be able to consider how your story addresses or fails to address said issues. Personally, I would be worried if someone I knew who was 18 was dating someone over 30, and if I was reading a book about such a situation and the book didn't take some issue with it, I would probably stop reading, but I can also see it being executed in a manner that wouldn't put me off.
Point 1: There is a difference between what is historically accurate and what is a good idea. Our ancestors were idiots, and a lot of things they thought or did (racism, genocide, monarchy, the earth being flat, so on and so forth) have since been discredited. Saying that just because it happened makes it okay is an is/ought fallacy, and it's not a legitimate argument.
It's strange and dumb to have historical characters mysteriously have modern mindsets (Victorian feminists did not look like modern feminists, so plonking some dude with a modern view of sex and gender identity in Victorian Boston would be unrealistic) but that doesn't mean that the book has to condone their outdated behavior. It can be a subtle line to walk, but one measure of it is to look at how what the story defines as real is different from what the characters think it is.
Point 2: I think that one major issue to look at in relationships being "creepy" is how the balance of power sits between the participating parties. No matter what age the participants are, having a romance between an office worker and their boss means that the boss could essentially blackmail the office worker into it, whether they do or not. And once that power differential exists outside of the relationship-- society places more power in the hands of one party than in the hands of another-- it's hard for it to not translate into a difference of power within the relationship, and that's starts messing with consent, because one party is more able to refuse consent than another.
And, as a rule, older people have more power than younger people. They've had more time to work their way up the meritocracy, to develop ethos with those around them, and to build any sort of network of influence. There are some situations where a younger person would have more power than an older one, but they are in opposition to the trend. So, with a few exceptions, a difference in age automatically transfers to a difference in power, and that's not good for a relationship sitting well with your audience.
Point 3: You'd have to get more into psychology to really flesh this out, but there's a good reason for the taboo against parent-child romance: the roles of the parent/child attachment and romantic partnership are contradictory in some ways. And, as a mentorship takes on some aspects of a parent/child relationship, having it evolve into a romance would leave one or both participants needing to act in two different ways at once. And no, I can't make that any more vague.
Point 4: Our ancestors were dumb, but kids are dumber. The reason that, legally, kids under 18 can't consent (I'm talking US here) is that they really aren't capable of the reasoning to make that decision. And in fact, it isn't until 25 that the parts of their brains that do that reasoning are fully developed. Add to that the lack of experience on which to draw, and the fact that they're more likely to let someone older do their decision-making for them, and we get back to Things That Mess With Consent.
So why is it not creepy when all parties in a romantic relationship are underage? Well, if it is a mistake, there's no one who should have known better. And there's it issue of everyone has about as little power as everyone else, again. And kids don't plan for the long term: very few high school romances are expected by any party to result in lifelong marriage or partnership. They make a mistake, they learn from it, they move on. There's a certain learning curve, and after about 25-30, that stops being true.
So, could you write a relationship with a wide age gap that addresses all these issues or has some reason for them to not apply, or you could write it with the knowledge that it is kinda creepy and messed up. Things don't have to be perfect for you to write about them. But see the above re: writing about things that are historically accurate without necessarily condoning them.