Scientists say this may help explain why childhood autism rates are rising.
The study in the journal Nature looked at Icelandic families with children diagnosed with autism or schizophrenia. It finds a father's age was crucial to the genetic risk of such disorders.
Researchers suspect, as a man ages, the number of hereditary mutations in his sperm increases.
Experts say the finding is not reason enough for men not have children later in life, but it could influence some reproductive decisions.