Left to its own devices, your immune system supports a fairly fixed upper limit of T-cells, and the fraction of those cells that can respond effectively to new threats, or patrol the body to destroy senescent or precancerous cells, declines with age. On the one side ever more T-cells become specialized in futile attempts to deal with persistent but otherwise not terribly threatening viruses such as CMV, and thus become useless for other activities. On the other side, the new supply of T-cells dwindles to nothing as the thymus atrophies - a process called thymic involution, something that happens comparatively early in life.
So a range of possible approaches could be taken to restore a sufficient number of aggressive T-cells to the body. Using stem cell technologies to create large numbers of a patient's own T-cells and then infuse them periodically, for example - that's possible today, but only being tried in trials for specific named diseases, as is usually the case for things that might be beneficial to all old people. Hopefully overseas clinics and medical tourism will pick up the slack. Other approaches involve destroying the unneeded specialized cells to free up immune system capacity, or trying to increase the supply of new immune cells by at least partially regenerating and restoring the thymus.
The thymus is a thumb-sized organ just above the sternum where our immune cells are trained to recognize self from other. It is fully developed by the time we are 10 years old, but after that it begins gradually to shrink. By age 25, it has already lost 30% of its mass, and by age 60 it is less than half its peak size. There is evidence that this is related to the immune decline that contributes so much to growing mortality risk with age, and that reversing that decline might lead to longer, healthier lives.
Research on reversal of thymic [degeneration] is a backwater of medical science. If this is an opportunity for major gains in life expectancy, then it is a neglected opportunity that has attracted little interest or funding. Based on evolutionary arguments, the general attitude seems to be that if the thymus shrinks over a lifetime, then it must not be much needed; or, conversely, that a Law of Nature assures us that any therapy to maintain its function must necessarily have dangerous side-effects that outweigh the benefits. [But] this is ideology, a misplaced faith in general theory over explicit experimental results. Reality in the lab appears to be that: "Thymic involution [seems] to provide no obvious benefits in humans that would outweigh the benefits of [its] elimination once the hazards associated with such issues as insulin-like signaling can be set aside."