You could make it work. The older kids take care of the younger kids, perhaps supervised by robot nannies, until they're old enough to be released from the nurseries at sixteen. They then get to enjoy five years of nonstop hedonism, with no responsiblities and all their needs provided for until they turn 21 and are considered obsolete. New infants are produced in test tubes.
It's MTV's "Real World" as a futuristic dystopia. What could be more relevant?
Casting twenty-plus adults kind of misses the point. "Never Trust Anybody Over Twenty!"
With The City fully automated, releasing them as young as 10 would hold up. As portrayed in the original novel.