- Men and women both reach physical maturity after going through puberty. Puberty is a growth phase characterized by extreme increases in height, hormonal levels and sexual interest. In males and females, puberty may be accompanied by symptoms such as acne and mood swings. The most widely accepted theory about puberty is that the changes seen during these years are a consequence of a rapid increase in the level of sex hormones in the body, which has a difficult time adjusting to the sudden introduction of new chemicals. Differences between male and female puberty (such as muscular development versus breast development) stem from the fact that men have more testosterone while women have more estrogen.
- Sexual development is a consequence of physical changes that occur over the lifespan of men and women. Contrary to popular belief, sexual development does not end with puberty. In men, the development of sperm begins to decline around age 25. In women, the production of eggs stops around age 50. While men and women never completely cease to be sexual, sex drive definitely declines with age. One hypothesis for this phenomenon is the "grandmother hypothesis," which states that adults benefit more from raising children than having new children after a few decades of fertility.
- Senescence is the phenomenon of living organisms naturally starting to lose proper bodily function and eventually dying. The processes by which an organism begins to break down are not entirely understood by scientists, but the contributing factors (organ wear, neural deterioration, loss of muscular strength) are all known. In almost all countries, females outlive males. The most accepted hypothesis regarding this phenomenon is that many disorders will occur only in an organism if present on both sex chromosomes, and men, whose sex chromosomes are identical, are more likely to get a genetic disorder than women, whose sex chromosomes are different.
- Men and women show different signs and symptoms as they age. Men are far more likely to experience age-related balding than women, while women are more apt to experience a sharp decrease in sexual interest than men. While signs such as heart disease, hearing loss and impaired mental functioning are common to both men and women, they affect men and women at different rates. The most widely accepted theory for these differing aging signs is that they are the result of different sex hormones present in men and women. Male baldness, for example, is caused by a hormone called dihydrotestosterone, a sex hormone that sometimes causes hair follices to decay and die.