Site of RBC formation in a 20 year old healthy male is:

Erythropoiesis : 1st trimester - in mesoderm of yolk sac (Mesoblastic stage); 2nd trimester - liver & spleen (Hepatic stage); 6 months onward - bone marrow (Myeloid stage). At bih - all bones have red bone marrow, producing cells. Progressively, bone marrow turns yellow; shafts of long bones first turn yellow, followed by their ends. By 20 years - red marrow exists only in sternum, ribs, veebrae, pelvis and skull.