it's not just the raw numbers, but the numbers that survive. if you look at infant mortality rates and the death of the mother - both happen more with younger mothers. if you don't care if the woman dies, or if the baby does - sure. that's also one reason in Europe, the nobility had wet nurses - to churn out as many babies as possible. but normal people nursed their own babies, so they would have a baby only every couple of years, typically.

if you care about the health of the mother and also the overall health of the child (vs. just having it survive but maybe be retarded or have other health issues) then waiting until the mother was in the early 20s makes sense.