The paternal age effect can refer to the statistical relationships of: (1) a man's age to sperm and semen abnormalities; (2) a man's age to his fertility; (3) a man's age to adverse pregnancy outcomes in his female partner (including miscarriage and fetal death); (4) a father's age at the birth of his offspring on the probability of an adverse birth outcome (such as low birthweight); or (5) a father's age at the birth of his offspring on the probability that the offspring will have a health-related condition (e.g., decreased intelligence), a specific disease (e.g., autism spectrum disorder, bipolar disorder, Down syndrome, and schizophrenia), or an increased risk of mortality.

The genetic quality of sperm, as well as its volume and motility, all typically decrease with age. Scientists have formulated at least two hypotheses to explain how paternal age might cause health effects.