There were some in universe reasons but those were put in by the author and she could’ve made the age limit 16 or 18 just fine if she wanted to.

I found it real icky at first, then they said it’s just a political marriage and I was fine, then they suddenly fell in love anyways and i got grossed out again. Also her constantly having to defuse this short fused bomb that is Hadis constantly threatening to fuck up the whole world on a slight misstep of the people around him. And you know the worst part? The defusing him in the final episode happened by her promising to bear him 10 children.

I don’t know why I sat through it all but I regret it.