Starting with sedentary mice aged about 65 in mouse-years, half were put on a program of high-intensity interval training:

the interval-trained mice seemed in many ways younger than they had been at the start. In particular, they were stronger; when pulled backward gently by researchers, they would cling to a bar longer than at the start of the study. They also had greater endurance capacity, as well as more muscle mass in their hind legs than the sedentary animals, and they scampered faster. Few now were frail.

Source: High-Intensity Workouts May Be Good at Any Age – The New York Times