The relationship between bone age and actual age (referred to as age) can be indicated by the bone age difference. The bone age difference is the difference between age and bone age, which is the specific number of years that the two differ by. A positive bone age difference means that bone age is behind age; a negative bone age difference means that bone age is ahead of age. Usually, ±2 years is the normal range of bone age difference, where a bone age difference within ±1 year is considered normal. A bone age greater than 1 year but not more than 2 years is considered early; a bone age less than 1 year but not more than 2 years is considered late. If the bone age is more than 2 years behind the age, the bone age is considered abnormally behind; if the bone age is more than 2 years ahead of the age, the bone age is considered abnormally ahead. The average annual increase in bone age is approximately 1 year, with fluctuations between 0 and 2 years. If the bone age increases by more than 2 years in 1 year, it indicates that the bone development rate is too fast. If the rate of increase in bone age is greater than the rate of height growth, the epiphysis will heal earlier and the growth period will be shorter, eventually resulting in a decrease in adult height. The first menstrual period is an important sign reflecting the maturity of a girl’s youth development, and the bone age and the first menstruation have a close time relationship. The average bone age at menarche is 13 years old, with a range of 12 to 14.5 years, so bone age is often used to predict the time of menarche. In addition, bone age is also closely related to the sudden increase in height during puberty than age.