One of the reasons that the adaptive immune system declines with age is that too much of its limited resources become devoted to uselessly chasing persistent herpesviruses like CMV. But there are other mechanisms at work too - not just a depletion of cells ready to act, but a decline in these cells' ability to act.
So we have research such as this, in which scientists chase down age-related molecular mechanisms that hold back the effectiveness of immune cells, and try to reverse them:
Circulating T-helper cells fall into two broad categories. "Naïve" T-helper cells have never encountered an antigen before (as in the case of, say, a rare or emerging pathogen or a new vaccine), but are capable of wheeling into action once they do. It takes a week or two to reach full tilt. "Memory" T-helper cells have previously been exposed to an antigen. These cells are long-lived and narrowly fixed on that particular antigen. They can rapidly transition to an activated state should the same antigen ever cross their path again. That's why prior exposure - through infection or a vaccine - renders us more resistant.
[Researchers] showed that faulty regulation in memory T-helper cells, due to aging-related increased levels of a protein called DUSP4, inhibits the activation of those cells, with their consequent failure to ignite a good B-cell (antibody-producing) response. This time around, the investigators uncovered a similar effect with a related protein, DUSP6, on naïve T-helper cells. In test tubes, they compared blood cells drawn from people ages 20-35 versus 70-85 in response to stimulation. In naïve T-helper cells (but not in memory cells), there were age-associated differences in a specific chain of biochemical events involved in the cells' activation, proliferation and differentiation. Laboratory tests showed that the culprit behind the cells' fecklessness in older people was DUSP6, [with levels that were] much higher in older people's naïve T-helper cells.
Further experimentation revealed that DUSP6's increase in aging naïve T-helper cells was caused by an age-associated easing up on a brake pedal called miR-181a, one among hundreds of small molecules made of RNA (called microRNA) that regulate proteins' production. ... Artificially boosting miRNA-181a levels in naïve human T cells caused DUSP6 levels to plummet, commensurately increasing those cells' readiness to activate on exposure to a given dose of influenza vaccine. In contrast, artificially increasing the levels of DUSP6 blocked the beneficial effects of heightened miR-181a levels.