Researchers are making inroads into showing that stem cell decline with aging is a function of the surrounding environment - you might recall the experiments in which old mice were given young blood, for example. Here is another research report: "Increasing studies have demonstrated the importance of extrinsic cellular factors on the aging of adult stem cells. Aged mouse spermatogonial stem cells have been transplanted into young recipient hosts for over three years without any decline in function. Serum from old mice markedly induces embryonic stem cell dysfunction. However, the effects of the aged environment on [mesenchymal stem cell (MSC)] senescence and function have not yet been reported. In the present study, the young and the old systemic milieu were mimicked by adding 20% [young rat serum (YRS) and old rat serum (ORS)] into the culture medium respectively. The results show that the ORS culture clearly promoted senescence and [reactive oxygen species] production in the MSCs compared with those cultured with YRS. The proliferation and survival ability of the MSCs were also significantly inhibited in the ORS group compared with that in the YS group. Therefore, ORS induces MSC senescence, as well as inhibit their proliferation and survival ability."