Correct, there's also link that shows that stem cell count decrease with age.
Excerpt
Aging also affects MSCs in humans and in animal models as indicated by the decrease in the bone marrow MSC pool and also shifts their lineage differentiation from one that usually favors osteoblastic differentiation to one that prefers adipogenic differentiation[35], which is largely responsible for the gradual and aging-associated shift of hematopoietic (red) marrows to fatty (yellow) marrows, and which also contributes significantly to the etiology of senile osteoporosis.
Could it be that it's not the number of stem cells that declines, but it is the effect of stem cells aging that leads to their inefficiency? Perhaps they age because they begin to accumulate genetic mutations and DNA damage over the years.
Are you alluding to
telomere shortening and
epigenetic changes? These theories have held for quite some time now to explain cell aging. For example, the progressive shortening of telomeres leads to
senescence,
apoptosis, or oncogenic transformation of somatic cells.