Few things about that: first, it's pivotal to the plot, since it's the reason for Aravis' flight, but it's also specifically commented on as being unusual, and explained as part of that culture. Second, in other times and places people getting married at 15 or 16 wasn't all that unusual. Even in colonial and pioneer America you see people doing it. Remember, for much of history the average age was about 45, so there wasn't that much time to waste. Thirdly, the story never has Aravis and Ahosta even meet, so it all stays in the realm of discussion. There's no reason anyone should be "creeped out" about it.