I found this excerpt from the book "Exceptionally Gifted Children" by Miraca U.M. Gross:
Age of Parents at Birth of Subject Child (Ch 5, p 78)
The literature on intellectual giftedness suggests strongly that the gifted tend to be children of older parents. In Terman's sample (Terman, 1925), the mean age of fathers at the birth of the subject child was 33.6 years, with a median of 32.6 years. The mean age of the mothers at the children's birth was 29 years with a median of 28.5 years. In the mid-1980s, Rogers (1986) reported that the mean age of mothers of children of average intellectual ability was 25.4 years. By contrast, in Silverman and Kearney's Colorado sample of children of IQ 170+ (Silverman and Kearney, 1989) the mean age of mothers at the time of the child's birth was 29.6 years, while VanTassel-Baska (1983) reported that in her sample of highly gifted finalists in the 1982 Midwest Talent Search, the majority of subjects were born when their fathers were in their early thirties and their mothers in their late twenties.
The author mentions these as the "American studies" and goes on to describe a later Australian study with similar results.
I sent you a PM with a link (it's very long) to the excerpt.
