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.
In the cryotubes aboard the Botany Bay
, we clearly see three men and two women, all adults, and glimpse four others who seem to be of approximately adult size, putting them in their late teens at least. Later, when Khan revives his people, we see four adult men and three adult women (though the blonde in the back looks kind of teenagerish). We also see four men, probably the same four, with him in engineering.
Spock said that after the war, "there were some eighty or ninety of these young supermen unaccounted for when they were finally defeated." I suppose he could've been counting children, but they would've had to be preadolescent when they fled. Not out of the question, I guess. But it's annoying that the inconsistency exists in the first place.