- age by generation or age reached in the year: difference between the year considered and the year of birth of the individual,
- age in completed years or age at last birthday. In the same generation, the age in completed years is not the same for all people.
For example, an individual born on 10 October 1925 dies on 18 April 1999. He is 74 years old in age reached in the year: 1999 - 1925 = 74. But he is 73 in number of years passed: 18 April 1999 - 10 October 1925 = 73 years 6 months and 8 days.
As such, for an individual who is x years old in age reached in the year, if the event takes place on date d:
- the individual having his birthday after the date d will have his age in number of years passed (x-1);
- the individual having his birthday on the date d or before will have his age in number of years passed (x).
The only exception: the event takes place on 31 December. On this date, the classification by age reached in the year and by age in years passed are the same. And on 1 January, the age reached in the year is equal to the age in years passed plus one.