@Eclat123 koreans has a unique way of counting their age. The moment they are born, they are 1yo, including the time they spent in the womb.
Also, their age didn't increase on their bday. Instead, they get 1 year added on 1st january every year. So if they are born on 31 dec for example, the will be 2yo the next day. If I am not wrong.
From this, safe to assume that the twins was born before 1st january. So they are 20 in their korean age (18yo international age). 20+5 = 25