It's extremely rare for healthy adults to have iron deficiencies in developed countries. Blood iron levels are linked to cancer, diabetes, heart disease and Alzheimer's.[1] Particularly among populations of European descent, where 5-10% of the population carries at least one allele for hemochromatosis.
Generally the ideal iron level for adults is as close to clinical anemia as possible. Randomized control trials have shown that regular phlebotomies (i.e. donating blood) significantly cuts the chance of developing diabetes and cancer in European populations.