Lately, we’ve been hearing about tech moguls who are spending millions in search of the secrets to longevity. But I’ve found my own fountain of youth, and it costs almost nothing. It’s not some superfood, or miracle supplement, or cryogenic treatment. At 84, I wake up each day feeling young, energetic, and ready to take on a busy day. And the number-one reason for that is strength-training. At 70, I was a physical wreck and on the fast track to old age.