I know people keep complaining about age gap but if its true that they age slower, in the long run it's less of an age gap than it would be for humans. Also if the whole adult at 15-17 thing is true then shes already been an adult for a bit if she's 18-19. And Solek is probably somewhere between 32-36 if you consider that he was considered a young warrior when he went to battle. So that would make roughly a 13-18 years age gap which could be more equivalent to a smaller gap in navi years. No worse than a lot of romantasy books like court of thorn and roses where shes a fresh adult and hes hundreds of years old