I get that all, but it still creeps me out. But 6 years don't feel right. She would be 12 when he was 18, and some you had take care of you when you young and you end up marrying them is still kinda manipulation, even if they are both of age because of the interaction when she was so young. Even if it wasn't intentional, she became emotionally dependent on him at a super young age, and my guess is will not get the chance to date around or anything. And they are already both being lovey in my eyes, even if they dont realize it.
I know it supposed to be cute fansty stuff, but this is sadly something that happens in real, often life in much darker ways. :(
The age difference is a given, but it can happen. What get me is he has known he since she was a Child. A child, taking care of her. I know the intent it good, but it still off putting. I wouldn't use the word grooming, but i can't help but want to drop this if they end up together.