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Populations can be divided into three ecologically important age classes or stages: prereproductive, reproductive, and postreproductive.
A biologist might look for the wear and replacement of teeth in deer and other ungulates, growth rings in the cementum of the teeth of carnivores and ungulates, or annual growth rings in the horns of mountain sheep. Among birds, observations of plumage changes and wear in both living and dead individuals can separate juveniles from subadults (in some species) and adults. Aging of fish is most commonly accomplished by counting rings deposited annually (annuli) on hard parts including scales, otoliths (ear bones), and spines.

The approximate ages of trees in which growth is seasonal can be determined by counting annual growth rings (Figure 8.17), a procedure called dendrochronology
Such assumptions, it was discovered, were valid for dominant canopy trees; but with their growth suppressed by lack of light, moisture, or nutrients, smaller understory trees, seedlings, and saplings add little to their diameters. Although their diameters suggest youth, small trees are often the same age as large individuals in the canopy. Attempts to age nonwoody plants have met with less success.
Once the age structure of a population has been determined, it can be represented graphically in the form of an age pyramid