I don't recall any dialog establishing that.
There was nothing in the film that established that.
In Greg Cox's novel To Reign in Hell: The Exile of Khan Noonien Singh, they're claimed to be the swiftly-grown children of the originals, but that's never asserted in the movie. All the movie tells us is that these 20-something people were stranded 15 years earlier, which doesn't add up.
Herbert(s) you are stiff!
I wish more people would remember that TMP and TWOK were retconning the appearance of Starfleet tech and costumes four decades before Discovery did it.
Because that's what Herbert wants. For everyone to be stiff like he is.
Oh, it was a dumb decision. I'm just trying to make the best of it from a fan standpoint and conjecturing.
The less we try to pick it apart the better off we'll all probably be.
That's the way Herbert works, he gets in your mind.
Just like Saavik is or isn't Half Romulan or a whole bunch of other things, the character
https://memory-alpha.fandom.com/wiki/Joachim was supposed to be Khan's son at one point, even if it was cut.
I'm not here to bandy about what's "official" for a 35+ year old movie. It makes more sense than no explanation but then, no one ever said that all of Khan's frozen 72 followers were the same age as he. It could have been 25 adults and their kids. Maybe the Ericksons had a lot of kids. Someone like Khan or some of those other supermen may certainly have had "harems" and many children is not that much of a reach, the children staying in with many of the adults falling prey to the "Ceti Eels"*.
KIRK: The others?
SCOTT: There's no change, and they're mixed types. Western, mid-European, Latin, Oriental.
KIRK: How many alive?
SCOTT: Twelve units have malfunctioned, leaving seventy two still operating. Thirty of those are women.
Those are the only descriptions in the episode
*The name "Ceti eel" is never referenced during the
movie Star Trek II: The Wrath of Khan. Khan describes the animal without naming it, and McCoy doesn't recognize it when it exits Chekov's ear, though Chekov had told Kirk and McCoy of it when they were on
Regula I.
And I'm joking, mostly, please don't be so stiff you're offended by being called Herbert, you bunch of humorless Vulcans!