Its a very complex Question, and its answer can be even more complex ... unless you specialize in the study of Hematology.
So the practical answer is ... as we age, the physical body's ability to regenerate its losses, meaning, as it functioned, in the prime ... gets weaker.
The bone marrow, primarily is the source of Red Blood Cell generation ... When we are old, this capacity reduces with the passage of time... hence, into various weaknesses, called diseases.
The best course in such conditions is to strengthen the bones, and take light, digestible supplements in our food, to assist revive/improve the marrows functioning. Meaning, take Calcium and Iron containing Vegetables ... with iron's supplementary pills ...
This is no prognosis, do consult your physician, regarding your person specific, condition.