Good point.  Fertility declines between 35 and 40.  It goes off a cliff between 40 and 45.  I know a couple who didn't start trying until the wife was 37.  They had to go the IVF route.  They spent about 75K on various fertility treatments and finally had a child at 42.  Then they spent more money trying for a second child but had no success.

If this couple really wants kids, they should make that the first priority.  It's always possible to work a few years past 65 to save more money for retirement.  But once a woman's too old to have kids, there's no way for her to get her fertility back.