While the decrease in male fertility with age is not as drastic as with female fertility, men are likely to be more fertile when they are younger
It is well known that advancing age of the female partner makes it more difficult to get pregnant, as well as increasing the risk of miscarriage and chromosomal abnormalities in pregnancy such as Down syndrome. But the association between advancing age of the male and fertility outcome is not as clear.
Older men often have older female partners, so it is sometimes very difficult to determine the effect of advanced paternal vs. maternal age. Older men also tend to have sexual intercourse less frequently than younger men, so this must be considered when trying to determine the effect of paternal age.
Semen volume has consistently been found to decrease with paternal age. Studies looking at sperm count and motility (what percentage of sperm is moving) have shown variable results. While counts may decrease with age, more study is required to confirm this.
Advanced paternal age has been associated with some rare birth defects. For example offspring of men older than 50 have been found to be 8 times more likely to have achondroplasia, a form of dwarfism. There is also evidence that advancing paternal age may increase the incidence of autism or schizophrenia.
Older aged male partners have also been associated with lower fertility rates and increased miscarriage rates. This seems to hold true even when adjusting for the age of the female partner and the frequency of sexual intercourse. It has also been found that IVF rates are lowered with advancing paternal age.
So as with women, men are likely more fertile when they are younger. While the decreased efficiency of reproduction with aging is not as marked as it is in women, there is still an effect that should be considered when an older man wants to reproduce.