Definition: A forest or stand containing trees of almost all age classes up to and including trees of harvestable age.

An age class is one of the intervals, commonly 10 or 20 years, into which the age range of tree crops is divided for classification or use.

Also pertains to the trees included in such an interval. For example, trees ranging in age from 21 to 40 years fall into a 30-year age class; 30 designates the midpoint of the 20-year interval from 21 to 40 years.