What is the most important factor in staying healthy through the years? Diet or exercise?

Both! That said, i favor exercise to maintain health. Regular exercise has many health benefits that are independent of weight. Meaning it is better to be overweight and fit, than normal weight but out of shape. I always push exercise first. Once that is a part of the regular schedule, work on healthy eating habits. Diet should include healthy foods and be in moderation. Hope that helps!