Plenty of people from my mother's generation (including my mother) seem to say that if they could go back and do it again, they'd have waited longer before having children. Maybe not necessarily to their 30s, but certainly later than the early 20s that they did. I think there is a feeling that they missed out on a lot of experiences that they could have had, where waiting another 5-10 years would have cost them nothing reproductively.
We can probably come up with an optimal window where the parents are in a good situation economically, reproductively, relationship is stable, grandparents are still available etc. but it would easily become too narrow to be achievable or too wide to be of any importance. The situation you describe
ending up badly is what the "cautionary" ticking clock stories are about. Something like: "Okay here is the deal I want to enjoy life while I'm still young so children are on the table maybe 5 years from now." And the man says "ok, babe". Five years later: "Honey, I've had five years to think about it and I'm not yet ready to be a father."
It seems to me -- as we talked in the v1 thread not long ago -- that the inequality between sexes as young adults is linked to the issue somehow. The
optimal dynamic between partners isn't the same between sexes either which was a disillusioning surprise to me, because it wasn't something I believed in.
About what experiences one truly misses extensive traveling seems to be the only definitive one. I guess it's enough to push the average age for young parents a few years further...