I believe the AAP guidelines refer to full term, healthy infants. For exclusively breastfed infants iron supplementation should start at 4 months and continue until solid foods that are iron fortified are started after 6 months of age. The infant is born with an iron store from the mother lasting about 4 months. Formula fed infants can be fed formula with iron.
Premature infants don’t have much of iron store and need supplement from the start.