Pediatrician James Tanner first developed a series of five stages to chart male puberty. The Tanner Stages, as they are called, now serve as common reference points to denote the changes a young male undergoes during adolescence. The exact age at which these changes occur depends on the individual, but typically they occur between the ages of 9 and 18.
At the first Tanner Stage of male puberty, changes are comparatively small. The boy grows by an average of 2 to 2 1/2 inches a year, he has trace amounts of pubic hair, and his penis and testicles begin to slowly lengthen. Erections are infrequent, but do occur. This stage takes place at about the age of 11, though it can occur anywhere between 9 and 13, depending on the child.
Changes accelerate during the second Tanner Stage, which usually takes place between the ages of 11 and 13. The boy experiences an increased number of erections, and the skin of his testicular sac thins. Pubic hair becomes thicker and darker, and the remainder of his body becomes lean. His testicles lengthen to about 1 to 1 1/4 inches, accompanied by a reddening of the surrounding skin. During this stage a boy grows about 3 inches per year.
At stage three, which usually occurs around age 14, the boy's penis continues to lengthen and his testicular sac grows by another 1/2 inch. Pubic hair is now quite prominent, and his nipples may become more sensitive. The boy's voice usually begins to break during stage three, and muscle mass becomes increasingly prominent.
By the time the boy reaches age 15 or so, he is growing by an average of 4 inches a year. His testicles lengthen by another 1/2 inch and his penis begins to thicken considerably. Underarm hair starts to appear and his pubic region is now fully covered in hair. Acute acne becomes common as glands in the skin swell and secrete oil. The boy's voice deepens as well, taking on the pitch and timbre of an adult man.
In the last Tanner Stage, age 16 to 18, the changes of adolescence are completed. The penis and testicles attain their full size and overall body growth slowly stops. Testicle length halts about approximately 1 3/4 inches, and pubic hair spreads to the boy's inner thighs. Hair is now growing on the boy's chin and cheeks, and muscles continue to develop and fill out the rest of the body.