@Miausi I see your point, kinda. I really do.
But I've really grown tired of this argument... You people REALLY need to stop applying the rules of OUR world to OTHER worlds. In Song of Fire and Ice Daenerys was like 14, wasn't she? Did you hate that too? Tyrion's father hired him a prostitute when he was 13, no? Did you hate that too?
I really, really, REALLY hope the answer is "yes" to both, because then at least you'll be consistent. But it's still moot. In THAT universe, with a medieval age setting, people had sex at 13. Deal with it. Political marriages were a thing. Deal with it.
Doesn't mean you have to "like it". I don't like James Bond being a murderer and using women like they're tissues, but well, it's the character, so I accept it.
Plus, you're actually wrong. There's such a thing called "will" or "motive" (I think there's a different legal term, too). In this setting, a 50yo man/woman would ALSO be "grooming" the young lady to, surprise, turn her into a decent noble lady. The MC is not really different. He's really NOT trying to seduce her from a young age. He's trying to raise her into a proper lady so she can marry her original romantic target. IF, and that's an IF, she happens to fall for him... that's by no means his doing. And IF, and that's another IF, he also happens to fall for her (once she's older and legal, and not a 9yo child)... well, I guess you'd say "he should properly reject her". Maybe he should, morally, from the viewpoint of OUR world. Which, again, it's not that world.
And still not "grooming".