Geometric representation of age structure is a characteristic of

A population has certain attributes that an individual organism does not have, such as birth rate, death rate, sex ratio and age distribution.
A population at any given time is composed of individuals of different ages.
If the age distribution (percentage of individuals of a given age or age group) is plotted for the population, the resulting structure is called an age pyramid.
The age pyramid is thus a geometric representation of the age structure, with the pre-reproductive, reproductive and post-reproductive groups shown one above the other.
The shape of the pyramid shows whether the population is growing (triangular), stable (bell shaped) or declining (urn shaped).
So the geometric representation of age structure is a characteristic of a population.