It isn't so much that women are "most fertile" in their 20s. It's that between 20 and 30 is the time least likely to wind up in either miscarriage (on the young end) and mental retards (on the old end).

Frankly, I think the preponderance of retards has at least something to do with too many couples waiting until they're in their 30s.

And yes, there's evidence to suggest that the older a man gets, the more likely it is that his sperm is going to result in a retard, too (copying errors get more common as you age).

But of course, the retard-lovers of the world don't want to talk about that angle, because they don't see a problem with retards (even though they're usually the same people most offended at the idea of being related to other apes.)