From publisher blurb:
Issue #2: Monsters That Improve with Age
An age chart can be a wonderful tool for Game Masters who want to scale monster encounters so that they are commensurate with the powers and artifacts possessed by the player characters. A chart of this sort is made up of three columns, one each for the age of a creature, its hit dice, and notes. Notes include damage, mutations, drawbacks, and anything else that affects the creature in or out of combat.
There are a few alternate forms for these charts. Most have Hit Dice that increase by one per age level, a few do not. Some have hit dice that constantly increase, and some decrease after a certain level to represent the weaknesses of aging (I call these “elder age” charts). Some creatures can have multiple age charts, which represents changes in diet, habitat, proximity of other specific creatures, or exposure to certain elements, such as heat or radiation. In most cases, once a creature has started its path down one chart, it is restricted to that chart for its entire life. In rare cases a mutant can change growth charts but this means they have to survive exposure to an extreme of something, which could include radiation, chemicals, parasites, or disease. And, finally, there are those mutants that go through metamorphosis and have one or more age charts (e.g., a larval form might have one age chart and the adult form another).