They say age ain't nothing but a number.

Twenty-one, twenty-six, thirty-one--what's the significance? Is age merely just a number, or a baseline for seasonal performance? How can we quantify this?

Aging curves--also called aging patterns or age factors--show the relative performance of a group of players for each age, usually either showing how much (in percentage terms) a statistic improves or declines from one age to the next, or how the production at any age compares to the peak age.

One method used for aging curves is to simply add up the stats for every player at each age and look at the resulting sums. This is flawed, however; it doesn't account for the fact that there are many more players at age 24 than at age 34--the difference in yards or touchdowns per attempt won't offset the disparity in attempts.

You can solve this problem by dividing the sum by the number of players at that age, right? Well, technically, yes, but it's...