Henderson's Dictionary of Biological Terms, by Eleanor Lawrence. 10th Ed.
Evolutionary Ecology, by Eric R. Pianka, 3rd Ed.
Many biological incidents change fairly orderly with age. Newborns feed on their mother's milk. Reproduction begins at puberty, but the reproductive rate for those who have just reached puberty is not the same as the reproductive rate for older adults. Nor does every generation reach puberty at the same age. 20 years ago, haddock, an east coast fish, spawned at age 3, but today, 1-year-old haddock are having offspring. There are many other biological events that change with age besides reproduction, such as food choice, habitats can alter, and mortality changes with age.
Mortality curves are common graphs used to examine changes in a population's dynamics. Graphs showing the number of individuals per age shows how a population's survivorship alters with age. Graph 1 shows the percentage surviving from an initial birth of individuals, followed by the number who survived year by year.
Examining Graph 1, you will see that from birth to 2 yrs, a 20% drop in the number of individuals occurred. From 2 yrs to 70, the number of individuals remained fairly constant, then continued dropping until 100 yrs. So mortality and survivorship curves can be used to determine how well a population is doing, and how many individuals reach puberty, and contribute to the next generation.