It is well known that increased maternal age has a negative effect on egg quality, but recent research has shown that advanced paternal age is associated with low-quality sperm and genetic abnormalities. In fact, the British Andrology Society recommends that sperm donors should be no older than 45.
Men who had kids aged 30 and above were more likely to have kids with schizophrenia. It is not certain what causes this increased risk, but many researchers think that epigenetics plays a large role. The risk for bipolar disorder only increases much later compared to schizophrenia, at age 55 for the father. However, there is some controversy here as some studies have not found a correlation between high paternal age and bipolar disorder. People with bipolar disorder have greater levels of DNA methylation. Since DNA methylation increases with age it is a good area to investigate and see if that is the cause of the increased risk.
Fathers 35 and older have a 63% higher chance of their kids having some kind of cancer in childhood compared to fathers having kids 25 and younger. Scientists think shortening telomeres could play a role here. Autism is another disorder that is associated with increased maternal and paternal age. Here men greater than 45 have 3 times the chance of having an autistic child. The most likely cause for an increase in autism as fathers age has to do with mutations in transcription factors which, which are proteins that regulate genes.
Overall, as men age, their sperm mutates more and more which increases the chance of many genetic abnormalities such as autism or schizophrenia. Telomeres also decrease as one age, and more research needs to be done to see the effect of this on cancer rates of children born to older fathers.