One thing we should do is look at examples of guys that have stayed healthy and fit into old age and find what they have in common. Interestingly, there are a lot of superficial variations in diet and exercise.
For example, Art DeVany has his system -- randomly scheduled intense lifting, lots of walking, paleo diet, and picking up chicks -- and it's obviously worked well for him. Jack LaLanne is going strong in his 90s, and he eats a vegetarian diet and exercises two hours every day, almost the opposite of what DeVany recommends. Clarence Bass is similar in some ways to DeVany, but has his own little peculiarities.
Despite the differences, we can still find a lot of common ground between these approaches.
First, consistency. DeVany, LaLanne, and Bass all became interested in fitness as young men and stayed with it for their whole lives. We all know that it's easier to maintain than to gain, so building a good base while you're young and then maintaining as much of it as possible as you age is a very good strategy. Of course, people starting at an older age can still make good progress, but if you're young, you want to take advantage of what you have now and train for the long term.
Second, body composition and maintaining lean mass. This argues for some kind of resistance training as a regular part of your program.
Third, a diet of natural foods, with no sugars or processed products. Every health and longevity star I've heard of follows this rule, regardless of the differences between their diets.
Basically, I'm not sure there's a Master Plan to guarantee a long and active life, but there are general principles that have proven successful. The trick is finding an implementation of the principles that you can follow for decades.
"The enlightened never cease forging themselves."
-- Morihei Ueshiba