Physical development starts at different times and progresses at different rates in different people. So, the beginning of the development that comes with puberty varies from person to person — and that's completely normal.
The earliest physical change of puberty for girls is usually breast development, which most often begins around 10 or 11 years. But it's perfectly normal for breast development to start anytime between the ages of 7 and 13. A girl's first menstrual period usually occurs about 2 to 2½ years after breast development begins.