The average 13-year-old boy wears pants ranging from size 12S to size 14S. Children grow very quickly and their measurements may change substantially over a period of months, meaning that knowing average sizes for a given age is a good way to keep ahead of the buying curve.
Size 12S corresponds roughly to a 24-inch waist and a 28-inch length. Size 14S suggests a 25-inch waist and a 28.5-inch length. The sizes scale incrementally rather than exponentially or sequentially.
To measure a boy's waist size, use a measuring tape approximately 1 to 2 inches above his natural waist, where his pants normally sit. The length should be measured from below the crotch area to just below the ankle.