I'll say it if Qam won't. The vast majority of childless 30 something and 40 something individuals I meet are bigger children than my children. And by children I don't mean charmingly playful or youthful. I mean self-centered prima-donnas with little or no maturity.
I was just shy of 38 and my husband was 43 when our daughter was born...........
Correlation is not causation. There are plenty of 30-something and 40-something individuals with children who are "self-centered prima-donnas with little or no maturity". (These people are the worst IMO, treating their children as baubles.)
The ability to get knocked up has no bearing on maturity or character. Where I live, being childless in your 30s and 40s is normal, and not remotely a mark of maturity.