I vote both nature and nurture. Nurture from the older mother vs a potentially less well equipped younger mother.
Nature in that the women who have children with older men may not on the average be as intelligent as the women who have children with same age men. I say this because a 60 year old women is not normally able to have children so a 60 year old man must be having children with a significantly younger woman.
Without adjusting for the mother and father's IQ I don't think this is a very valid observation.
I know this is anecdotal but the only women I know who had children late in life were well educated and appeared intelligent.