Witold Bialokur...can run 10 kilometers, or 6.2 miles, in less than 44 minutes. While Mr. Bialokur's performance would be the envy of most young men, he is not young. Mr. Bialokur is 71. It is one of the persistent mysteries of aging, researchers say. Why would one person, like Mr. Bialokur, remain so hale and hearty while another, who had seemed just as healthy, start to weaken and slow down? Rigorous studies are now showing that seeing, or hearing, gloomy nostrums about what it is like to be old can make people walk more slowly, hear and remember less well, and even affect their cardiovascular systems. Positive images of aging have the opposite effects. The constant message that old people are expected to be slow and weak and forgetful is not a reason for the full-blown frailty syndrome. But it may help push people along that path.