Men can't say, as women can, that a particular event -- the cessation of menses -- brings them into life's next stage. Physiologically and even psychologically, men really do not go through a "male menopause." Their midlife hormonal changes are not only more gradual but also less extreme.

But some changes in a man's middle years can be expected. Among them:Sexuality. Men in their 40s and 50s take a longer time than younger men to become sexually aroused, a longer time to reach orgasm and require a longer time after orgasm to become aroused again. The angle of their erection declines by nearly 20 degrees, and the number of sperm released with each ejaculation declines, too.Skin and bones. As in women, men's skin begins wrinkling and their bones begin thinning some time in the 30s. But there's a difference: Men start out in life with skin and bones that are thicker, so the losses are less significant.

Heart disease. The biggest age-related health risk in men is the risk of cardiovascular disease, especially atherosclerosis and heart attack. The male risk of heart disease is higher than the female risk ever becomes, even after women's risk is elevated with the decline of estrogen after menopause. But while estrogen has a protective effect for women, it has no similar effect for men. Men who take estrogen for medical reasons -- primarily for the treatment of rare cancers -- still have a higher risk of heart disease.The reproductive system. The closest a man comes to having ovaries is his testicles, which, like ovaries, secrete the bulk of sex hormones. Testicles do not shut down the way ovaries do, but they do secrete less testosterone as a man ages. When this happens, a secondary sex organ, the prostate gland, enlarges. In many men, this enlargement can interfere with urination and must be treated surgically.