The human sexual response has been described as a cycle with four stages—desire, excitement, orgasm, and resolution—and in men, a refractory period during which they cannot be restimulated to arousal. The sexual response cycle is not simply a mechanical chain of events, however; it involves specific biological responses to psychological and sensory inputs. Thus, the transition from one stage of the human sexual response to another is not automatic, even if it is stereotypical. The human sexual response also depends on important biological events. First, there must be vasocongestion, a process in which an increased amount of blood concentrates in the tissues of the genitals. Second, muscle tone must increase. For these two processes to occur, the nervous system must function appropriately, and there can be no significant peripheral nerve impairments. If these biological processes cannot take place because of injury or illness, a person may have sexual feelings, but these feelings may not lead to a classical sexual response, such as vaginal lubrication or full penile erection.

Aging may be associated with changes in sexuality. Many changes begin before the fifth decade of life. In men, significant changes in erections and ejaculation occur with age. As men get older, it usually takes more (not less) stimulation to achieve an erection. In addition, the phase of sexual excitement is longer (not shorter) and may not end in ejaculation. When ejaculation occurs, the force of expulsion of semen and the intensity of ejaculation are lower. After ejaculation, the erection resolves more quickly, and the refractory period increases. Many of these changes start gradually and can begin when men are in their 40s. Age-associated changes should be anticipated and viewed as normal. Although sexual interest declines to some degree with aging, older men and women who live together are more sexually active than those who are not in relationships. Because women tend to live longer than men, many elderly women are without partners and have limited opportunities for sexual expression, other than masturbation, even if sexual drive is present.