Lots of good advice on here.
I'm just in my 70s now. I don't drink or smoke. Eat very healthily and try to keep active, both in body and mind. It's really all you can do.
A lot of it is down to genes. I don't expect to live into really old age as I come from a long line of people that tended to die in their 70s possibly early 80s if they were lucky.
I know someone though, who is in the lower half of his 90s now and is still pretty healthy. He's still living in his own home, still tends his garden and walks to get his newspaper each day. His mother lived till almost 102 and died very quickly - her body just sort of shut down, it was a very good way to go.
Keeping your critical thinking 'muscles' healthy, as pp suggested, is very important, that, and not becoming a daily mail reader, or a watcher of rubbish day time television.
Keeping your mind young and open to new ideas will make you willing to do more new things, and learning new things is vital to keeping the mind active.
You can't stop yourself getting old, but you can stop yourself mentally 'sinking' into old age.
Physically, it's not so easy, sometimes our health is out of our hands.