This type of question represents one of the things that really turns people away from getting into science, or makes people leave. You are expected to give up your ENTIRE PERSONAL LIFE to "the good of society" and make these sacrifices, for...what? To get denied tenure in 5 years (if you're lucky enough to be tenure-track)? This is even more true for women, though this isn't unique to academia. I was thirty years old, had been married for 6 years, just finished my PhD, and thought starting a family sounded like a reasonable thing to. And, my next two kids were born after my postdoc. So, fuck that, and anyone who thinks that waiting until you're 40 to have kids sounds reasonable.
The last time I looked at science there was no vow of staying childless required.
Waiting for kids until you're 'settled' just isn't a justifiable demand for a career that takes 30 years to become established.