I mean, the inherently interesting thing about such premises is that they test our definitions of what is acceptable—which part of an age-gap, and/or underage, romance is morally troubling? We agree (well, most of modern society agrees) that it is problematic, but usually not on why.
Is it, literally, that certain physically mismatched bodies in a romance are in some inexplicable way immoral? No? Is it the disparity in knowledge (in which case a reincarnator might be fine)? Or in mental, or emotional maturity (in which a reincarnator might be in hazy territory)? In the overall power imbalance, both physical and influential, between an adult and a child (a reincarnator is arguably on shaky ground here)? Some combination thereof—if so, which bits are relevant and why?
Furthermore, is there a broader exception to norms, that we want to advance, here? Is it always morally okay to keep loving someone so long as it was socially acceptable to love them when you started loving them? Is the alternative—having to give up on who you love because of circumstances beyond your control—terrible, or does it even matter if it's terrible?
(To be clear, the fact that such premises pose such questions, doesn't give works carte blanche to avoid scrutiny; quite the opposite. I mean, if we weren't scrutinizing the right and wrong of it, then that part of it wouldn't be interesting. And of course, you can have a setup that is obviously just a thin excuse, to try to "get around" the moral issue of pederasty in a work of... questionable taste and merit.)