@Abedeus
That's actually a bit more complicated than that.
There are 3 differents notions, and I feel you are mixing them :
- Age of majority : Age when you are considered an adult and free to do anything related
- Age of consent : Age when you are considered able to consent having sex
- Marriageable age : Age when you can marry
Most countries have an age of majority at 18. A few have in under that (up to 14 for Iran !), and some have it over (up to 21).
Age of consent is generally lower. Many developped countries have it around 16, sometimes even 15. BUT generally at the condition it's not with someone having an authority over you (teacher, boss, etc...)
Marriageable age is generally around 18 (age of majority) in most developped countries, BUT it can also be lower with parental or judicial consent, around 16 mostly.
So I don't think that you'll find many countries where marriageable age is under age of consent. Which means that once LEGALLY married, there's absolutely no problem with having sex (legally). Would be actually weird otherwise.
As we are talking about this manga, in Japan, even if age of majority is at 20, a girl can marry from as early as 16 WITH parental consent. And she can have sex as early as 13 until it's not with someone with authority ! And with anyone she wants from 18.
I guess the fact he's way older can make a grey zone in term of authority, but he has no recognised authority over her.
To sum it up :
- They can already have sex together, even if this could be a bit of a grey zone/dangerous. At worst they'll totally be free to do it within a year at most.
- They can already marry together if her parents consent to it
Note that it's only a LEGAL discussion. I'm not talking about morality here, which is another one.