Is there a benefit for exercise in my 80s?

There is. But the goals of exercise in your 80's will be very different compared to that of in your 30's. Exercise in your 80's can be greatly helpful for balance issues, maintenance of muscle bone, maintenance of bone density, good for optimizing moods, just to name of few benefits.
Absolutely! Exercise at any age can be very beneficial. You can incerease muscle mass, aerobic capacity, bone strength and balance, sense of vitality, improved mood, improved memory. Just start slowly and carefully. If you have not been exercising, you should see your doctor and make sure there are reasons to avoid exercise. If it is safe for you to exercise, have someone who knows how to, help you start.