There was a story on Hacker News about this a while back, but here's something I found through a quick search.

Best age for childbearing remains 20-35 - Delaying risks heartbreak, say experts http://www.medicalnewstoday.com/articles/30737.php

Yes, and I don't think it's easy for women to have children at 60 or 10, however it's still possible. Biologically it's probably optimal to have children in the 16 - 22, socially 25 - 35 is probably ideal. However, I was simply pointing out that the limits of biology are well outside the shift in optimal range we are talking about.

Men might be able to have children at 60, but it's not so easy for women.