Mesenchymal stem cells (MSCs) residing in bone marrow (BM) are the progenitors for osteoblasts and for several other cell types. In humans, the age‐related decrease in bone mass could reflect decreased osteoblasts secondary to an age‐related loss of osteoprogenitors. To test this hypothesis, BM cells were isolated from vertebral bodies of thoracic and lumbar spine (T1–L5) from 41 donors (16 women and 25 men) of various ages (3–70 years old) after death from traumatic injury. Primary cultures were grown in alpha modified essential medium with fetal bovine serum for 13 days until adherent cells formed colonies (CFU‐Fs). Colonies that stained positive for alkaline phosphatase activity (CFU‐F/ALP+) were considered to have osteogenic potential. BM nucleated cells were plated (0.5, 1, 2.5, 5, or 10 × 106 cells/10‐cm dish) and grown in dexamethasone (Dex), which promotes osteoblastic differentiation. The optimal plating efficiency using BM‐derived cells from donors of various ages was 5 × 106 cells/10‐cm dish. BM‐derived cells were also grown in the absence of Dex at this plating density. At the optimal plating density, in the presence of Dex, the number of CFU‐F/ALP+ present in the BM of the younger donors (3–36 years old) was 66.2 ± 9.6 per 106 cells (mean ± SEM), but only 14.7 ± 2.6 per 106 cells in the older donors (41–70 years old). With longer‐term culture (4–5 weeks) of these BM cells in medium containing 10 mM β‐glycerophosphate and 100 μg/ml ascorbic acid, the extracellular matrix mineralized, a result consistent with mature osteoblastic function. These results demonstrate that the number of MSCs with osteogenic potential (CFU‐F/ALP+) decreases early during aging in humans and may be responsible for the age‐related reduction in osteoblast number. Our results are particularly important in that the vertebrae are a site of high turnover osteoporosis and, possibly, the earliest site of bone loss in age‐related osteoporosis.