The site of RBC formation in a 20 year old healthy male is

The bone marrow of essentially all bones produces red blood cells until a person is 5 years old. The marrow of the long bones, except for the proximal portion of the humeri and tibiae, becomes quiet and produces no more red blood cells after about an age of 20 years. Beyond this age, most red cells continue to be produced in the marrow of the membranous bones, such as the vertebrae, sternum, ribs and ilia. Even in these bones, the marrow becomes less productive as age increases.