While this might be right for overall humanity, for white collar professional workers specifically (typical audience of HN), I think late 20's / early 30's is a

While this might be right for overall humanity, for white collar professional workers specifically (typical audience of HN), I think late 20's / early 30's is a pretty good time to have kids - people have a solid 10 years of career experience under their belt, proportional amount of savings which help with both a financial cushion and with down payment for a house and some emotional maturity. On the other hand, one can get all sorts of experiences in 20's and their novelty starts wearing off. Finally, one is still young enough to have enough energy for child rearing.