Genetics, psychological stress, and nutrition contribute to the age at onset of puberty. The average age at menarche is 12.75 years but it was much higher 150 years ago, when nutrition was poorer. The amount of fat stored in the body will help to determine whether puberty occurs in girls. It is believed that leptin, a hormone made by fat cells, helps to determine the age of menarche. Girls who are thin and active will have delayed menarche. The secondary sex characteristics develop at the time of puberty. In males, these characteristics include increased laryngeal size, which deepens the voice, greater muscle development, and the growth of pubic hair, facial hair, and overall body hair. In females, it is increased fat deposition in the hip and breast area, increased breast size, and broadening of the female hips. Pubic and axillary hair develop. The growth spurt in girls happens prior to the onset of menarche with breast development being the first sign. In males, the first sign is the growth of the testes. The first fertile ejaculate happens at around 15 years of age.
THE BRAIN AND SEXUALITY The brain is involved in all aspects of sexual behavior, from the onset of desire to orgasms and the need to cuddle. Researchers have done neuroimaging studies on human sexual behavior, which looks for the changes in the brain during sex in men and women. There is a four-component model that has been developed to try to understand the role the brain plays in sexual responsiveness. The first stage is cognitive or the thinking phase. It involves the perception of the visual sexual stimulus, deciding if it causes sexual feeling. The ventromedial prefrontal cortex in the front of the brain is connected to the limbic system, which is the emotion system of the brain, as well as certain parts of the brain associated with the senses. The end result is the focusing of the attention toward the sexual stimulus, which activates both the occipital lobe, involved in vision, and the temporal lobe, involved with hearing. There is a specific extrastriate body in the brain that is connected to perceiving the human body.
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