Exercise may prevent future stress which in turn lessens the effects of aging.
Obviously, exercise reduces stress at the time you are doing it. But reducing future stress means that exercise creates a preventive factor lasting well after the workout ends, though how long it lasts is not known. But it seems to act as a strong buffer against being stressed out and suffering all the side-effects that come with stress later on in the day.
So make hitting the gym or going for a run a regular part of your future stress busting routine.