The traditional age in China is nominal age, which is calculated according to the lunar calendar. At birth, it is 1 year-old, and it increases every Spring Festival 1 year-old. This year is 15 years old.
According to the Gregorian calendar, one year old is 0 years old, and every birthday increases 1 year old. Now it is 13 years old, and this September 15 will be 14 years old.