In 2e, 1e and OD&D, an 18 was genius.
Genius is 140 IQ on a standard deviation of 15, or 160 on a standard deviation of 22.5 (inferior tests). Anyway, a little less than 1 in 200 have such an IQ.
Take notice that 1 in 216 people, rolling 3d6, would get an 18. This isn't a coincidence :-p...