It’s complicated. I think when a girl is that young, the older man moulds them into their idea of a perfect partner (your brain isn’t fully developed until you’re 25). I’d imagine they were ‘taught’ early on not to argue and to be pliant and receptive to the older man (this is very apparent in Elvis and Priscilla’s relationship for example, although she managed to get away). There’s also the sunken costs fallacy, whereby you can’t admit something is wrong for you because you’ve invested so much time and energy into it. Also if you feel like the rest of the world is against your relationship, you double down on it because you feel like you have to prove everyone wrong (see Aaron Taylor Johnson and his grandma) and very often you’ve either isolated yourself from friends and family who disapprove or the opposite where friends and family have encouraged the relationship, so you have no one to turn to when you start to think ‘this is all wrong’. Nobody wants to admits that they were ‘stupid’ enough to be groomed so your relationship has to be the exception that transcends the norm annd it’s actually a love match and has nothing to do with age.
(I don’t think anyone who’s groomed is stupid by the way, but I imagine that if and when anyone breaks free from a controlling older person, there’s a lot of self recrimination).