They also told a conference that grip strength in middle-age was linked to the chance of a stroke.
The scientists said more studies were needed to understand what was happening.
Experts said the findings raised important questions, but more research was needed.
Suggestions of a link between slow walking speed and poor health have been made before.
A study, published in the British Medical Journal in 2009, said there was a "strong association" between slow walking speed and death from heart attacks and other heart problems. A Journal of the American Medical Association study suggested a link between walking faster over the age of 65 and a longer life.