One of the first noticeable secondary sex changes is a growth spurt in height. On average, the growth spurt for girls begins between the ages of 8 and 13 years; and in boys, between the ages of 10 and 16 years. Girls generally hit the peak of their growth spurt between the ages of 10 and 13 years, while boys hit the peak of their growth spurt between the ages of 12 and 15 years. Finally, young women reach their full adult height between the ages of 10 and 16 years, while young men reach their full adult height between the ages of 13 and 17.
Individual youth develop at different rates. These differences often result in some children feeling uncomfortable in their own skin on the basis of their looking differently than peers. This may occur if they grow taller at different times than their friends or peers. Likewise, youth may notice certain parts of their bodies, like legs or arms, growing at slightly different rates than other parts of their bodies. These periodic irregularities may cause them to feel more awkward and uncoordinated. However, in general, youth build muscle mass and experience an increase in strength and coordination during puberty.
Another obvious secondary sex change for both sexes is body hair. Youth begin developing pubic hair around their external genitals, or private area. Pubic hair appears for girls between the ages of 8 and 14 years, and for boys between the ages of 10 and 15 years. Pubic hair fully grows in between the ages of 14 and 15 years for girls and between the ages of 14 and 17 years for boys. Underarm hair appears in youth between the ages of 10 and 16 years. Boys begin developing facial hair between the ages of 12 and 15 years. In addition, all youth begin developing thicker, coarser body hair during this time. Head hair also becomes coarser and thicker and may even darken in color. Young men and women may become interested in shaving or otherwise grooming their facial, leg, underarm, or even pubic hair during this period. Their attitude toward grooming body hair is influenced by their peers and the customs of their family, religion, and culture.
Girls will also experience significant changes to their body shape while boys will experience a deepening of their voices. Girls may feel uncomfortable during puberty as their body shape changes to accommodate the future growth and development of a baby. These changes include a widening of the pelvic bones (or hips), an increase in body fat percentage, and the development of breasts. These changes may make some girls feel self-conscience or "fat," as they are unaccustomed to their new curvy bodies. Breasts begin to bud on average between the ages of 8 to 13 years. Breasts rarely grow at the exact same rate; thus, a girl may notice one breast is larger than the other during this phase. This is normal and expected but if breast asymmetry continues after breast development should be completed, families should consult a family physician. Breast growth is completed, on average, between 14 and 15 years of age.
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Meanwhile, boys experience their voice changing, or deepening. Their larynx, or voice box, grows larger and the vocal cord muscles grow stronger, resulting in a deepening of their voice.
During this process, which may take a while to complete, boys' voices may "crack" while they're talking. This is normal, but it can be annoying, or embarrassing to self-conscious youth.