The age when kids hit puberty varies wildly, according to KidsHealth. Girls can start puberty between the ages of 7 and 13, and boys can start puberty between the ages of 9 to 15, according to KidsHealth. Each child is an individual who will develop at her own pace. Even identical twins don’t necessarily develop on the same schedule, according to family physician Elizabeth A. Pector.
Puberty occurs when girls begin to secrete increased quantities of estrogen and boys begin to secrete increased quantities of testosterone. Girls develop breasts and fat deposits in the abdomen, hips, thighs and buttocks, while boys develop a beard and bigger muscles. Both sexes develop pubic and underarm hair. In addition, a boy’s voice deepens and a girl begins to have menstrual periods. Each child does this on her own schedule. Some girls develop breasts several years before their periods start, for example.
Girls and Age of Puberty
The average age of puberty in girls appears to be declining slightly, according to a January 2009 article in “The Daily Beast.” Increased body weight might be a factor -- weight and body fat affect estrogen production. A child who weighs more than her sister at the same age might start her periods earlier. Family structure also makes a difference. A girl whose birth father was absent from the home when she was younger is more likely to go through puberty early, although scientists are not sure why.
Birth Weight and Weight Gain
A child’s birth weight can affect the age of puberty in both boys and girls, according to research reported in the April 2012 issue of the journal “Pediatric Obesity.” The study examined 600 children from birth to adolescence. Birth weight was noted and weight gain calculated at 6 months, 12 months, 2 years and 5 years. Parents reported age of menarche in girls and signs of puberty in boys. The study found that girls with higher birth weight and increased weight gains reached puberty at an earlier age than peers. Heavier boys were also younger at puberty, but the effects were not as noticeable as with girls. A sibling who gained more weight as a young child could reach puberty earlier.
Twins are a special case. Identical twins are typically similar in terms of the timing of growth spurts, according to Pector. A smaller twin might not quite catch up by puberty, however, and might lag a bit in terms of physical development. Because girls usually hit peak growth earlier than boys, fraternal twins of opposite sexes are likely to reach puberty at different times. Fraternal twins of the same sex are no more closely related than any other two siblings and will develop at their own pace.