Why don't symptoms of sickle cell appear until children are around 4 months old?

Maturing hemoglobin: The hemoglobin involved with sickle cell develops after birth. Children are born with a different hemoglobin type and this changes over the first 3-6 months to be sickle hemoglobin.

Change in hemoglobin: Sickle cell disease is caused by an abnormality in adult hemoglobin (hgb). Before birth fetuses make fetal hgb; this works normally. At birth babies have mostly fetal hgb; as days go by the amount of adult (sickle) HGB increases and the fetal HGB decreases. At around 4 months the amt of sickle HGB is high enough to cause symptoms. Research is trying to find ways to keep fetal HGB level high.