An interesting article from the New York Times, written by Gretchen Reynolds, details a study by Dr. Michael Yassa from John Hopkins that revealed telling information about the role of the dentate gyrus in the hippocampus with regards to memory. Using advanced magnetic resonance imagery, activity in the dentate gyrus was measured when participants of varying ages recalled images from a week previous. Younger participants were able to distinguish subtle differences in the imagery such as whether or not it was new or only slightly different, such as whether a piano was a baby piano or a grand piano; older participants were not so good as this type of differentiating.
There has been numerous evidence that suggests in order to improve or aid our memories as we move into old age that exercise promotes neurogenesis in the dentate gyrus. A study by Pereira et al. (2006) demonstrated that neurogenesis does take place in the dentate gyrus of exercising adult mice. I have not come across any studies where this has been demonstrated in humans, unless anybody else knows better?