In the episode Space Seed in the original series, Khan is awakened from suspended animation, where he's been since the early 1990s. He is said/implied a product of eugenics, and his crew escapees from the eugenist side of the Eugenics War. He looks like he's got to be at least mid 30s, but possibly older since you'd assume a superman would age more slowly than most humans.
By my reckoning, this means Khan can't have been born any later than the 1960s. Which would mean the eugenists were already at work producing a super race at the time this episode was written. Did Roddenberry ever indicate he already thought organized eugenics were going on?