When it comes to maintaining cognitive abilities into old age, research suggests that a variety of factors are at play. The standard account of aging’s effect on cognitive functioning distinguishes between “fluid” and “crystallized” intelligence. Fluid intelligence—the ability to think and reason independent of prior knowledge—diminishes as we age. As early as our 20s, (or even earlier for some tasks), we can begin to experience a slow but increasing decline in the fluid aspects of our cognitive functioning, such as mental processing speed, working memory, attention span, and abstract problem-solving skills. By contrast, crystallized intelligence—the ability to draw on and apply one’s accumulated experience and knowledge—increases as we age, generally plateauing until sometime in our 60s and falling thereafter. In other words, the science confirms common experience: As we age, we gain wisdom within our range of expertise, but slow in our ability to operate effectively outside our comfort zone.

But what does the inevitable deterioration of fluid intelligence mean in terms of the ability to work into the golden years? The answer, unsatisfyingly, defies easy generalization. First, all humans approach the starting blocks of the race against time with different cognitive baselines. As Ian Deary, the director of the Centre for Cognitive Ageing at the University of Edinburgh, writes, “Of the many possible contributors to cognitive ability level in old age, none yet known approaches the effect size of mental ability measured in childhood.” To put it simply, smart kids generally make for smart seniors.

Second, the risk that one will suffer a neurological disorder that severely disrupts cognitive functioning increases dramatically as people age. The risk of stroke, for example, doubles each decade after age 55. The odds are equally unforgiving for Alzheimer’s, with the odds doubling every five years after age 65. The ability to remain productive as a person ages is therefore tied to the ever-increasing chance that his or her number will come up in the grim lottery of cognitive disease.

Research does show, however, that engaging in high levels of cognitive activity from childhood to middle age builds up a “cognitive reserve” that protects against the deterioration of mental functioning. This reserve inevitably dissipates as people age, but its effects can be long-lasting. Older people who continue to engage in mentally stimulating activities that require them to interact with and process information—such as reading the newspaper or playing chess—experience dramatically slower rates of cognitive decline than their idle peers. It should therefore come as no surprise that retirement represents a crucial setback in the battle to stay mentally fit, often prompting significant and sudden cognitive deterioration. One longitudinal study of nearly half a million people found that each additional year of delayed retirement reduces the risk of dementia by 3.2 percent. Similar results have been found for Alzheimer’s. And a recent study that examined brain autopsies shows that these effects occur even in people who have experienced the types of physical damage to their brains ordinarily associated with dementia and other neurological disorders—which suggests that continued mental stimulation leads to enhanced cognitive functioning, and not the other way around.