In the original book, the killing-people-at-21 policy was implemented specifically because originally the young people who implemented the policy preferred it to birth control. Kids under the age of 7 wouldn't be seen in public; the state raises them.

However, the way the society is *shown* doesn't make it look as though the birth rate is very high. I don't recall a large proportion of very young teens in the visible population.

Disclaimer: it's been a while since I read it.