@hina_sw Koreans consider a year in the womb as counting towards their age, so everyone is one year old when they're born. Then on new years, everyone will get a year added to their Korean age.
Basically, if you haven't had your birthday: add 2 years to your age. If you've had it: add 1 year.
The twins must've had their birthdays and were 18+1, plus their current lives would be 5+1. I.e. 25 years.
Well, I could be wrong though since it seems to be a mix of Korean age/"normal" age, but basically that's a bad explanation of their system.
I don't really think you can just add on to someone's mental age when they're in the body of a child, though, but that's just me lol