if they both have parents consent
No that is illegal.
An 18 year old girl can marry an 18 year old boy even if they are not in love in this state.
In most countries, no.
Of-course they can!
legally no that is basically robbing the cradle, if you know what that means, and why would you want to marry a 18 year old girl if your a 15 year old boy anyway?
No.
With parental consent, yes.
No, both are adults and do as they wish.
Not if the live in Norway.
With parents permission but seriously too young
Yes, if the minor has parental consent.
Possibly it depends on where you live and if the parents allow it.