Time lived since birth.
Age can be calculated in a number of ways:

Age in completed years: age at last birthday, i.e. number of complete years lived by an individual at a given moment in time.
Age reached: difference between the current year and the year of birth. It is used to define a birth cohort
Exact age: it measures the exact period of time elapsed since birth and therefore increases continuously. It is expressed in years, months and days, or in tenths or hundredths of a year.