@Arkos, it wasn't actually common in medieval times for 30 years old to marry 18 years old. It was just also frowned upon back then, and only people who viewed their daughters as tools to gain extra influence did it.
And yes, magic is a very big deal. Magic erases physical differences between men and women, so there would be no reason for patriarchal society in a world of magic, as female magician can be more powerful than male swordsman. So there would be literally no reason for a girl to marry someone she doesn't like, because she can be just as busy with her studies, and she can achieve the same results by herself, if she wants to.
So there is no point comparing actual medieval setting with magical medieval setting, as they can be fundamentally very different, and the only thing in common can be lack of technology. It can be a matriarchal society, if women are the only one capable of magic, for example, and then the rules can be VERY different. Or it can be virtually anything else. The only medieval thing about thing story is that there are no cars and planes, the culture doesn't have to adhere to medieval times.
So yeah, it's perfectly normal for family to be outraged that some random dude that is TWICE the age of their daughter wants to marry her.
If she was 16 and he was 23, it would be somewhat okay. If she was 12, and he was 17, it would be alright in some situations to arrange a marriage as well, since they are technically both children and 5 years age gap is not too big. E.g. if sir Conrad's family offered a marriage, that would be acceptable, as he is what, 16 or 17? And their family has similar social standing, and Leslie is acquainted with the guy himself, so there would be nothing wrong despite the age difference.
But some random 24 years old guy proposing is nonsense. This could even be viewed as looking down on ducal house. Whenever there is a marriage with big age gap, it was always the case that the bride's family was the one that benefited from it, since this was viewed as unfavorable match for a bride, so the groom had to be of higher social standing for it to ever happen. In those cases bride's feelings weren't even considered, the only purpose of such marriage was benefit.
It's very clear that the Duke house isn't the type to arrange such marriages, and most importantly, they care about Leslie's feelings a lot, so naturally they would be outraged.