Daily dietary requirements for iron vary by individual and are affected by factors such as growth periods for kids and menstrual cycles and pregnancy for women. The National Academy of Sciences set the Recommended Dietary Allowance for men and post-menopausal women at 8mg a day and at 18mg for pre-menopausal women. For pregnant women, it is 27mg, and for those breast-feeding and not menstruating, it is 9mg.
Because the average U.S. daily diet contains 12-15mg of iron, to prevent iron deficiency in high-need population segments, doctors often recommend iron supplements.