Immunosenescence is the steady degeneration of the immune system that occurs with age. For the adaptive immune system at least, researchers have a good picture as to why and how this happens - which means that they also have starting points to develop ways to reverse immunosenescence.
Here is an open access review paper on the topic:
The elderly frequently suffer from severe infections. Vaccination could protect them against several infectious diseases, but it can be effective only if cells that are capable of responding are still present in the repertoire. Recent vaccination strategies in the elderly might achieve low effectiveness due to age-related immune impairment.
Ageing dampens the ability of B cells to produce antibodies against novel antigens. Exhausted memory B lymphocyte subsets replace naive cells. Decline of cell-mediated immunity is the consequence of multiple changes, including thymic atrophy, reduced output of new T lymphocytes, accumulation of anergic memory cells, and deficiencies in cytokines production. Persistent viral and parasitic infections contribute to the loss of immunosurveillance and premature exhaustion of T cells.
In essence, the immune system fails because the thymus, source of immune cells, ceases production and withers away. At the same time, the population of immune cells becomes ever more biased towards memory cells and away from cells capable of fighting new infections - and this is largely due to persistent viruses like cytomegalovirus. Eventually the immune system becomes so focused on the viruses it cannot clear from the body that it has no resources left to perform its other functions.
These problems suggest their own solutions: replace or rejuvenate the thymus, for example, or apply new targeted cell-killing methods developed for cancer therapies to destroy unwanted memory cells. On that note it's worth recalling that thymus transplants have been shown to extend life in mice, but there is plenty of other evidence to support these and similar attempts to restore the aging immune system to youthful levels of activity.
Ongrádi, J., & Kövesdi, V. (2010). Factors that may impact on immunosenescence: an appraisal Immunity & Ageing, 7 (1) DOI: 10.1186/1742-4933-7-7