As you report, the research by Darren Baker of the Mayo Clinic in Minnesota has shown that mice can be made more vigorous in old age without prolonging their life (“Forever Young?” November 5, 2011). Making the ageing less arduous has to do with the removal of cells that have reached the so-called Hayflick limit in cell division, after Leonard Hayflick, a biologist who discovered it. Once cells reach this limit, they cannot divide any longer, but they malignly affect neighboring cells that are still capable of division. This research points at the possibility of making humans, too, live more vigorous lives in old age without prolonging their life. In humans, the Hayflick limit is about sixty divisions. Removing the cells that have reached it may be possible, just like in mice. As you point out, this would work with the grain of nature rather than against it. However, there is also a possibility of extending the periods between cell divisions, which you do not mention. It would not breach the Hayflick limit, but it would stretch the amount of time needed for each of the sixty or so divisions. Together with the removal of the cells that have reached the limit along the lines of Baker’s research, this would not only make people more vigorous in old age, but would also prolong their lives while still working with the grain of nature. Of course, this possibility would lead to yet another limit, and that is the maximum amount of time between cell divisions. Some biologist is sure to discover it sooner or later.