In the first stage of puberty (which often occurs sometime between the ages of 8 and 11), the only signs of puberty are occurring inside the body, as ovaries react to hormones such as LH and FSH and begin developing estrogen-producing capabilities.
The second stage of puberty generally takes place when a girl is 11 or 12. Breasts begin to grow, or "bud." Buds exist when the areola darkens and rises and some transitional breast tissue exists. This generally comes before all other visible signs of adolescence, though it's not uncommon for pubic hair to begin growing before the breasts begin to develop. A growth spurt will occur.
In the third stage of development, growth of the breasts and pubic hair continues. Hair may have begun to show up in the armpits, and the vagina grows in size. This stage usually covers a girl aged 12 or 13 (though any girl between the ages of 9 and 15 could also pass through this stage). By this stage in development, menarche will occur for seven out of 10 girls [source: LiveStrong]. Once the body receives its signal from the brain to undergo puberty, it takes less than three years for menstruation to occur.
In the fourth stage of puberty for girls (often around ages 13 and 14), the accelerated growth of her height will usually slow down. The adolescent's level of body fat will reach higher adult levels, tapering off at around 26 percent body fat. Pubic and underarm hair will continue to grow fuller and more coarse. If the first period has occurred, ovulation may be irregular as it establishes its rhythm. While it's now common for girls at 12 or 13 years of age to have already had their first periods, it wasn't always so. In fact, just over 100 years ago, most girls were 15 when they had their first periods (changes in nutrition, health and the environment are believed responsible) [source: Stoppler].
Stage five represents the fully matured girl (usually around ages 14 to 17, though not uncommonly up to age 19) who has passed through all preceding stages of puberty. The girl has likely reached her maximum adult height by this time. Breasts have likely reached their full size, and pubic hair is fully developed. Periods and ovulation occur regularly. The cardiovascular, skeletal and muscle systems are all fully developed.