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.
Wrong. There is nothing on this earth more sobering or more maturing than when you take responsibility for a new life. The childless can deny this until they are all blue in the face but it won't change the veracity of it at all.