Marrying age for aristocracy in ye olde days was about 14. Younger for girls, actually.
But asking that for a pixel game is pointless. Not only is it fiction, it is pixels. An example for fiction without pixels is Romeo and Juliet. She was 13 in that play.
So yeah, if it were any way realistically, if the heroine of the game is unmarried, she probably is way less than 18, as are all the other aristocratic heirs that are unmarried.
But since this is a fictional world, let's just imagine they are on a planet that has about 30% less time per day, so she and the other heirs actually are 21, just to be sure.
My point is, they are legal in the context of the game world. If they were not, it would be a plot device. It has no bearing what is currently arbitrarily legal age in real world. And even there is difference where you live. There are places where it is 21. And then there is age of consent, that goes as low as 13 or up to: must be married. Oh, and in Japan it interstingly was 20, lowered to 18 in 2021 (age of majority).