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.
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.