The American Journal of Epidemiology reports on research indicating that the risk for autism increases with maternal (>35 years of age) and paternal age (greater than 40 years of age).  There was also a greater risk of autism for first born children to older parents.  This study looked at 1251 8 year old children with a diagnosis of autism in 1994. 

Reference:  Maureen S. Durkin , Matthew J. Maenner , Craig J. Newschaffer , Li-Ching Lee , Christopher M. Cunniff , Julie L. Daniels , Russell S. Kirby , Lewis Leavitt , Lisa Miller , Walter Zahorodny , and Laura A. Schieve Advanced Parental Age and the Risk of Autism Spectrum Disorder American Journal of Epidemiology Advance Access published on December 1, 2008, DOI 10.1093/aje/kwn250.

Am. J. Epidemiol. 168: 1268-1276.