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.
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.