** Definition **

The 3rd quartile (Q3) is the age above which the 25% of the oldest population is located.

It is calculated from the population by age group.

Each age group is assigned an age which corresponds to the middle of the class.

** Example **

The age group from which the cumulative population is greater than or equal to 75% of the total population.

In our example counting 30 inhabitants, the 3rd quartile is in the age group 5-9 years because the cumulative population in the lower age groups does not total 22.5 inhabitants * (30 * 0.75). *

The 3rd quartile is therefore, according to the age assigned to each age group, 7 years.