Several studies have demonstrated a decline in the male reproductive system, sperm quality, and fertility with advancing paternal age, yet many of the biological mechanisms that underlie this process remain poorly understood. It is unclear whether the problem arises from the progenitor spermatogonial stem cells (for example, from an accumulation of DNA damage and mutations), from the somatic niche present in the testis (consisting of Sertoli and peritubular myoid cells), or from a combination of the two. Current data, albeit from a small number of studies, suggest that both factors have a role in age-associated germ cell loss. What is clear, on the other hand, is that mounting evidence links paternal age to chromosomal damage and genetic problems in the children of older fathers. The frequency of de novo mutations increases markedly with age, leading to increased risk of breast cancer, cardiac defects, developmental disorders, behavioural disorders, and neurological disease in the children of older men. The current trend towards fathering children at a later age raises concerns regarding the risk of offspring developing complex multigene diseases.