their korean age are 18 and 22 yes, but i think the translation is now using the international age. Kang is 16 going on 17, while Irae is 21. the thing is: in Korea, not only that you're already 1 year old when you're born, your age also adds when it's the new year (no matter when the actual birthday is, usually it's only for celebration or like the official birthday)
so it seems that Kang is actually 16 going on 17 (international), but is considered 18 (his birthday hasn't come so even though he's 17 going on 18 in Korea, he is already considered 18)