What exactly is hypocellular bone marrow?

Not enough cells: The bone marrow is the factory of the blood -- all blood cells begin life there. The "normal" amount of cells decreases with age, so we estimate normal as 100 - (age) +/- 20%. So, if a 50 year old has 50% of the space in the bone marrow filled with cells, we say this is normal. If it were 25%, it would be hypocellular. However, if a 90 year old had 25% cellularity, it is probably normal. Helps?