I saved it till the last point to keep your attention. I know you've heard it before, but it can't hurt to hear it again. Exercising regularly makes you less likely to develop diabetes, heart disease, dementia, osteoarthritis, depression and obesity. One study found that women who walk briskly for just 5 hours a week have a 76% higher chance of aging gracefully and healthfully, with less physical and mental impairment. Of course, if you are aging, you can't exactly run an hour a day or rock climb like you used to (if you used to at all). Look into low impact workouts like walking, resistance training, tai chi and yoga.
You can't control every biological process that goes on in your body as you age, but what you can always control is your outlook and the lifestyle choices you make. Turns out that's more than half the battle. About 66%, to be exact!