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.
- 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.