Doubling a child's height at age 2 can provide an close estimate of how tall that child will be in adulthood.
Another predictor of a child's adult height is parents' height or, more specifically, midparental height. You can calculate midparental height by adding the mother's and father's height in inches. Add 3 more inches for boys or subtract 3 inches for girls. Divide by two. Most children will reach an adult height within 2 inches of mid-parental height.