A boy's muscle fibers actually change during puberty.

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Later Male Puberty Stages, from Cracking Voices to Bulging Muscles

The third stage of male puberty occurs around age 13 or 14, but frequently as early as 11 or late as 16. Pubic hair begins to grow darker and fuller, and the penis now begins to grow in length. The testicles continue growing, erections become commonplace and the boy gains in height at a rate of over 3 inches (7.6 centimeters) a year. The voice may begin "breaking" or "cracking." This is when the boy's voice suddenly and uncontrollably changes pitch mid-word or sentence, due to the growth of the larynx and the lengthening of the vocal cords.

Stage four of male puberty commonly takes place when a boy is around 14 or 15 years old. Hair begins showing up in the armpits and on the face, and pubic hair begins to grow coarse. The boy's voice will even out and become deeper. The stage-four boy can use this deeper voice to yell forcefully at the mirror when he sees his new acne, due to his ever-more-oily skin. He begins to grow taller and even faster, at about 4 inches (10.2 centimeters) a year. The penis now grows thicker and continues to lengthen.

The final stage of male puberty occurs anywhere from age 14 to age 18. During this time, a boy will achieve most (but not necessarily all) of his height. His body shape will have evolved to that of a man's -- his shoulders will be broader, his muscles developed and fully formed, his arms and legs and chest proportioned for power and masculine appearance. This is because new muscle fibers are appearing, and all muscle fibers are getting thicker. Pubic hair will spread out to the inner thighs and lower stomach. By this stage of puberty, boys -- make that young men -- will be shaving, and their pubic hair and sex organs will look fully developed. While hair and height and body type will look adult and complete, many males will continue growing and developing into their 20s.

In the next section, we'll take a closer look at the growth process itself.