All I know is, I look at myself and I look at my family, and I can already see how much healthier I am than my siblings. I'm enjoying life in a way that they can't while they still eat junk. My sister is diabetic, my brother is morbidly obese. Neither one of them could stand up to my standard daily walk of 2 hours at around 2.5 mph. They get worn out easily, and they can't enjoy going to a museum or a zoo for the day, because it's just too hard on them. As long as I'm in such great shape, I see no reason to change things.
Look at Mark Sisson. He's almost 60 and in better shape than your average 18 year old. Paleo life expectancy was to age 54 (one you avoided dying before you were 15 - infant mortality and all that), and that was before modern medicine, proper sanitation, etc. It's really hard to know how long a Paleo type could have lived with the same advantages we now have. Heck, it's kept my SAD father alive to age 70, though not without developing heart disease and diabetes, which severely takes away from his quality of life.