** Definition **

The 1st quartile (Q1) is the age below which the youngest 25% of the 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 25% of the total population.

In our example counting 30 inhabitants, the 1st quartile is in the 0-4 age group because the cumulative population in this age group is greater than 7.5 * (30 * 0.25). *

The 1st quartile is therefore, according to the age assigned to each age group, 2 years.