One year old. Age refers to a person's life span from birth to calculation, usually expressed in years or years. Legally, it is calculated according to the actual age, which is one year old. The calculation method of one year old is the current year-year of birth = one year old. The nominal age at birth is one year old. According to the lunar calendar, you will add one year when you wait for the Chinese New Year. The calculation method is to add one year to the difference between a person's birth year and the current year. In math problems, the age problem is usually calculated according to the law, so it is generally one year old. For example, Xiao Min's cousin is 18 years old, Xiao Min is 5 years old, and how old is his cousin? The calculation method is: 18-5= 13 (years old).