To assist in the study of cellular aging, we established a new method of enriching physiologically aged bone marrow stromal cells (BMSCs) in animals of any age using a Percoll density gradient centrifugation technique. BMSCs from mice 2 months of age were isolated, and their cellular age determined (over 80% Scal-1(+) CD29(+) CD11b(-) CD45(-) CD105(-) and able to differentiate into osteoblasts, adipocytes, and chondrocytes). As proof-of principle, cells were aged in vitro and confirmed by low bromodeoxyuridine (BrdU) incorporation and senescence-associated beta-galactosidase (SA-beta-gal) staining. Proliferating cells were enriched in high-density gradient layers, and senescent cells were enriched in low-density gradient layers. We confirmed that over 80% of cells from the low-density gradient layer were senescent with SA-beta-gal staining and telomere dysfunction-induced foci (TIF) assay. This density-based method, which can separate proliferating and senescent BMSCs, could be used to study mechanisms of physiologic cell aging and may have implications for the use of BMSCs in clinical transplant applications.