In male mammals, changes at all levels of the hypothalamic-pituitary-testicular axis, including alterations in the GnRH pulse generator, gonadotropin secretion, and testicular steroidogenesis contribute to an age-related decline in circulating testosterone concentrations. The rate of age-related decline in testosterone levels is affected by the presence of chronic illness, adiposity, medication, and methods of testosterone measurement. Cross-sectional surveys reveal an association of low androgen levels with changes in body composition, physical function, risk of diabetes, coronary artery disease, and bone mineral density and fracture risk. However, the long term risks and benefits of testosterone replacement in older men with low testosterone levels remain unknown. Testicular morphology, semen production, and fertility are maintained up to a very old age in men. There is weak evidence of a small increase in the risk of specific genetic disorders among the offspring of older men. The clinical consequences of age-related changes in circulating testosterone concentrations and epigenetic changes in sperm DNA are poorly understood.