A group of individuals of the same age (year of birth) in a given population. Age group, a group of individuals of the same age in a population; a structural element of the population. The demographic structure of a population includes newborn individuals, young, semi-mature, mature, over-mature, old, etc. Many organisms are characterised by a simple age structure, where all individuals in the population are approximately the same age. Species characterised by long life spans usually form populations with complex age composition, and the characteristics of the age structure both favour and hinder reproduction.