18 세의 서양인 "을 한국어로?

The literal translation would be 서양 나이로 18살, but I doubt anyone ever says that. We usually say 만 18살 or 만으로 18살, and sometimes 만 나이로 18살 but this's rare. Example: - 얘 이제 몇 살이지? = How old is he now? - (한국 나이로) 세살이야. 만으론 2살하고 6개월 됐어. = He's three (in the Korean age), or two years and six months in the western age.