[lqdc13]

It might be that people enjoy living without children for as long as possible in the current western economic/cultural climate.

This age might keep increasing because those who try too late won't reproduce. So it's passive selection for longevity.

[golergka]

I think it's passive selection for reproducing in your late teens or early twenties out of bad luck. Even a century ago, your wealth level had a tremendous impact on whether or not your prodigy will survive, so there was an evolutionary selection for social success; with modern welfare state, I don't think that there's any selection bias towards the wealthy.

[philwelch]

Or passive selection for not being part of an infertile and decadent culture.