Quantitative as well as qualitative bone loss occurs with aging in both men and women, leading to alterations in skeletal microarchitecture and increased fracture incidence. Sex steroids, primarily estrogen and testosterone, have been shown to play a central role in the aging process of bone. The relationship between diminishing estrogen levels in women caused by ovarian failure and the development of postmenopausal osteoporosis is widely recognized. Unexpectedly, bone mineral density at various skeletal sites in men is also better correlated with circulating levels of bioavailable estrogen than with testosterone. Recently, it is also suggested that senescent osteocytes and their senescence-associated secretory phenotype may contribute to age-related bone loss. Osteoporosis should be considered as a disease developing on the basis of the natural aging process which is modified to some degree by various genetic and environmental factors.