Three recent studies point to the importance of cardiorespiratory fitness for older adults wanting to prevent cognitive decline.
Cardiorespiratory fitness improves executive function & episodic memory in older adults
A study comparing 33 young adults (age 18-31) and 27 older adults (age 55-82) has found that older adults with higher cardiorespiratory (i.e., fitness) levels performed as well as young adults on executive function tests. On long-term memory tests (a face–name memory task, and a visual episodic memory task), young adults performed better than older fit adults, who in turn performed better than less fit older adults.
Fitness had no effect on young adults' memory or executive functions (although bearing in mind other research, I'd suggest that this may be due to the cognitive testing being insufficiently demanding).
The finding confirms earlier research linking cardiorespiratory fitness with better executive function in older adults, and extends the association to episodic memory.
Regular exercise improves cerebral blood flow & short-term memory in older adults
A German study involving 40 healthy but initially sedentary older adults (aged 60-77) has found that those assigned to an exercise group (regular exercise on a treadmill for 3 months) improved not only their physical fitness but also their visual memory. This memory improvement was accompanied by increased blood flow and increased volume in the hippocampus. This improvement was seen in 7 of the 9 exercisers who were no older than 70. However, it tended not to be seen in those older than 70.
The control group (who also showed no such improvement) engaged in muscle relaxation sessions.
Increased brain perfusion as a result of physical exercise has previously been shown in younger people. This finding shows that some older adults may retain this ability, and also links this increase in blood flow to improvements in memory performance. It is less exciting to see that the effect was limited to visual short-term memory, but perhaps further exercise might have more far-reaching effects.
Cardiorespiratory fitness governs cerebrovascular health, not age
Another study has found that, among a group of older adults aged 55-85, blood flow in the gray matter was positively correlated with cardiorespiratory fitness and negatively correlated with age. That is, better cardiorespiratory fitness was linked to better blood flow in the brain, and both tended to decline with increasing age. Moreover, blood flow in the gray matter was entirely governed by cardiorespiratory fitness.
In other words, cerebrovascular health is largely a matter of your cardiorespiratory fitness, not your age.
 . Cardiorespiratory Fitness Is Associated With Cognitive Performance in Older But Not Younger Adults. The Journals of Gerontology Series B: Psychological Sciences and Social Sciences [Internet]. 2014 . Available from: http://psychsocgerontology.oxfordjournals.org/content/early/2014/12/19/geronb.gbu167
 Vascular hippocampal plasticity after aerobic exercise in older adults. Molecular Psychiatry [Internet]. 2014 . Available from: http://www.nature.com/mp/journal/vaop/ncurrent/abs/mp2014114a.html
 Cardiorespiratory fitness mediates the effects of aging on cerebral blood flow. Frontiers in Aging Neuroscience [Internet]. 2014 ;6. Available from: http://journal.frontiersin.org/Journal/10.3389/fnagi.2014.00059/abstract