By analyzing past age-at-burn patterns on a landscape, it is possible to quantify how strongly the hazard of burning is driven by the age of fuels. If the age of fuels can strongly limit or promote fire spread, this "age effect" should be evident from past burning patterns.

An age dependent system should be characterized by a fire frequency distribution that is more or less "bell shaped" -- that is, there is less burning in younger stands, an increasing probability of fires occurring in middle-aged vegetation, and not much landscape left to burn in older age classes.

In contrast, historical fires may exhibit burning patterns that are more or less random with respect to the age of fuels, not indicating any sort of dependence on the amount of time since the last fire.