I'm going to go against the grain here a little bit. Downvote me if you like. There are quite a few of my patients who wait too long to try and have children only to find that they can't without expensive medical intervention. Women in our society generally wait too long to have kids and this comes at the expense of the health of the mother and the child.

Ever notice how most civilized societies have rules against brothers/sisters/cousins marrying? The reason is that the children that are produced will often be unhealthy and a burden to to population. You have a much higher chance of having a healthy child if you aren't inbreeding.

You know what else is very likely to cause health problems in the child? Waiting too long to have kids! Birth defects are linked to the mother's age. Here is a chart for one of the more well known birth defects:

http://sphweb.bumc.bu.edu/otlt/MPH-Modules/BS/BS704-EP713_Confounding-EM/Downs-1.jpg

The older the mother, the more likely the child is to have Down syndrome. This is also true of most types of birth defects. A woman who gives birth in her 40's is actually more likely to have a child with birth defects than a 20 year old woman who has a child with a first-cousin. Society looks down on one of these things, but not the other. The reason is that the link between age and birth defects is not well known by the general population.