Birth cohort or age group: two ways to group people by age
A birth cohort is everyone born in the same year, followed over time. An age group is everyone of the same age on one date. The people born in 1990 are a cohort, the people aged 35 on 1 January 2026 are an age group.
Two questions, two groups
“How many people were born in the same year as me?” is a question about a birth cohort. “How many people are my age today?” is a question about an age group. They sound alike, but they group different people:
- A cohort keeps the same members all their lives. It only shrinks, through deaths, or changes through migration.
- An age group gets new members every year, as the next cohort reaches that age.
From one to the other
Population tables count people by completed years of age on a date. On 1 January of a year Y, the people aged a completed years were born between 2 January of Y − a − 1 and 1 January of Y − a. For example, everyone aged 34 on 1 January 2026 was born between 2 January 1991 and 1 January 1992.
So on 1 January the single age a matches the birth year Y − a − 1, shifted by one day. Anthrotempo uses this rule and calls that age group “born in Y − a − 1”. The one-day shift moves about one 365th of a year’s births and is mentioned wherever a result depends on it.
An example from the data
For 1 January 2026 the UN projects 124 million people in the world aged 35, which is the cohort born in 1990. Ten years later, on 1 January 2036, the same cohort is aged 45, and the UN projection counts 121 million. The difference is the cohort’s deaths and the net effect of migration, which is zero for the world as a whole.
Generations are cohorts too
Generation names such as Millennials or Generation Z are ranges of birth years, so they are groups of cohorts. Anthrotempo uses the boundary years of one cited definition and never describes what a generation is like.
Last updated 26 October 2026