Dementia and its evil twin, Alzheimer's, may have moved ahead of cancer on the list of most feared diseases, especially among baby boomers, who have begun to believe it is their inescapable fate if they have the bad luck to live too long.So we grasp at any news about aging, hoping that medical science has indeed found a way to preserve that most essential part of who we are -- our memories.Do we protect our minds by doing The New York Times crossword puzzle or by doing aerobics? By eating more leafy greens, absorbing more vitamin D from sunshine or memorizing poetry?"There is such a thing as normal memory change with age," said Dr. Susan Lehmann, who specializes in geriatric psychiatry at Johns Hopkins Hospital in Baltimore, "just as there are normal changes in vision." Our brain will not always work as fast, for example, and we won't be as good at multitasking. But these are all considered normal changes, according to Lehmann.