Let's say the old king has son at the age of 20. So if his son is 20 then he should be 40....

The old king likely had a son at the age of him going thru puberty and this could be anywhere from 12 to 15 years old. This was often common in the old ages. So yes it is a fact that in ancient times that a 30 year old man would be married to a 15 year old girl. This doesn't mean its not nasty for which I think it is. Yet if you want a historical story this is how history truly was. Keep in mind in some countries even today that follow some practices they men can marry a 9 year old girl. Its sick but it still happens in many middle eastern countries today.