I agree with those who mention exercise! It truly is the key. I remember a customer I had years ago. She was 70 years old, and ran marathons. She was as sharp and clever as you could imagine, and had the clearest eyes I've ever seen. I've experienced the magic of fitness myself over the past few years.
Beyond that, I read somewhere in just the last couple of days, that people who smile openly, are perceived as younger. Attitude would be the second piece of the equation in my book, possibly first to tell the truth.
That said, I hate aging. I feel like a youngster most of the time. I am happy to report though, that at the ripe old age of 47, I have had comments like "You get prettier every time I see you", " I card anyone who looks younger than 35", "No way you can be the mother of that 22 year old", and such like, all in the last couple of months. I attribute it to the smile, and a little bit of sunshine.