Eating less as you get older is actually a good idea. Your metabolism slows down as you get older. That's the main reason that many tend to put on weight later in life - their metabolism slows down, but they don't adjust their diet to suit.
I don't think there is any secret to living longer. Longevity is genetic. Short of some over-the-top, invasive, life support technology, there is nothing you can do to make yourself live longer than your genetics would otherwise dictate. But there is plenty you can do to shorten your life - eating poorly, smoking, not getting enough exercise, etc.