All cells of the body age and are replaced in a natural order. When RBCs age, they are destroyed in the spleen. During this process, the iron from their hemoglobin is released into the circulation and returned?

Bone marrow.

Explanation:

Hematopiesis may be defined as the formation of the blood cells by the precursor stem cells. This process occurs in the bone marrow, thymus and liver.

The red blood cells are enucleated cells with the minimum span of around 120 days. These cells are processed and destroyed in the spleen. The iron is recovered from the red blood cells and sent again into the bone marrow for the production of new blood cells.

Thus, the answer is bone marrow.