Puberty marks the time when hair starts to thicken or grow in areas where no hair grew before. While girls can expect hair to thicken on the legs and grow in the armpits and genital area, boys can expect thickened hair on the arms and legs, and hair growth on the face, armpits, genital area--and sometimes the chest, back and buttocks. The hair doesn't grow and thicken all at once but in stages that vary from person to person.
Stages of Male Hair Growth
Boys can expect puberty to begin between ages 9 and 15, when a small amount of pubic hair starts growing at the base of the penis. Between ages 11 and 16, boys can expect to see that hair gets darker, coarser and extends to the area between the legs and torso. Between ages 11 and 17, hair grows under the arms, on the chin and upper lip, and potentially on the anus. Between ages 14 and 18, pubic hair appears more as an adult man's hair. Facial hair begins to grow more, and into the early 20s, more body hair begins to grow in areas such as the chest and back.
Stages of Female Hair Growth
Girls typically begin to see the first stage of pubic and underarm hair growth between ages 8 and 14. Between ages 9 and 15, pubic hair begins to darken and become coarser. Between ages 10 and 16, pubic hair likely begins to take on a more triangular shape. Pubic hair should fill in between ages 12 and 19.
Factors Affecting Hair Growth
Luteinizing hormone (LH), secreted by the pituitary gland, stimulates puberty and the eventual onset of hair growth in boys and girls. LH stimulates secretion of testosterone in boys and estrogen in girls. Other hormones that initiate body changes are adrenal androgens in both sexes and progesterone in girls.
Early Hair Growth
Precocious or premature puberty causes a girl to begin seeing pubertal changes by age 8 and a boy to begin puberty by age 9. Although the direct cause of precocious puberty isn't always found, the cause is sometimes related to medical conditions such as hormonal disorders, infections, injuries and exposure to external estrogen and testosterone, such as through ointments.
Delayed Hair Growth
Although medical conditions such as dysfunctions in the pituitary thyroid and pituitary glands, diabetes, kidney disease, asthma and malnourishment can cause delayed puberty and associated hair growth, the cause is often genetics. Parents concerned about delayed puberty in their teen should seek counsel from a doctor.