If Khan and his band of multiethnic adult followers were stranded 15 years ago, how come all his followers are now white and in their 20s?
Because they are genetically perfect fifteen-year-olds, all born from the dominant marriage(s) on the colony?
Why the assumption that all of Khan's frozen "followers" were adults? In Space Seed, we saw a hand full of adult males, and Scotty reported that thirty of the followers were women.
And not all the followers on the Reliant were in their twenties, there was another man approximately Khan's age also on the bridge.
Timo, have you considered that the some of the people in the suspended animation chambers could have been children, most being pre-adolescent? This would account for the twenty-something characters we saw on the bridge.
SCOTT: ...and they're mixed types. Western, mid-European, Latin, Oriental. [later] Twelve units have malfunctioned, leaving seventy two still operating. Thirty of those are women.