If iron isn't excreted from the body, how can someone who didn't have anemia before become iron deficient anemic?

Iron IS lost: A woman during reproductive life loses a fair amount of iron each month, and everyone loses iron each day form skin, gut and so forth -- and a bleeding ulcer or cancer can announced itself by causing extra iron loss and anemia.

Menses, perhaps?: Not all anemias are due to iron deficiency. But many cases are. Women lose iron through menses or pregnancies. Men don't have this physiologic mechanism of blood loss, so in their case, iron deficiency anemia is due to bleeding (sometimes imperceptible) or poor oral intake (as in children).

Monthly cycle: Probable the most common source of anemia in a monthly cycle woman is there monthly period, especially if it is heavy and associated with clots. Check with you family doctor to discuss further. For men it would be abnormal to be anemia at age 25 and should be check out.