David,
The general guideline for computing marrow cellularity as "100-age=percent" applies even at advanced age. So, yes, a 90-year-old would be expected to have something in the area of 10% cellularity. In fact, the rate of decline in cellularity seems to accelerate after age 70. I have heard doctors describe the effects of MDS on cellularity as making it seem as though the bone marrow is just wearing out.
There is a chart on page 9 of
Bone Marrow Pathology by Barbara J. Bain, David M. Clark, and Irvin A. Lampert that can be seen at
Google Books. It shows the expected decline in cellularity of normal bone marrow as people age.
Hope this helps.
Ruth