I need an equation for 50+ flabby.

I never heard of "foxy grampa" but I still remember someone I met when I was 7 or 8, on a vacation at the Texas east coast. This guy had biked from the west coast and was still going. He was over 50 - I thought "ancient" at the time - but nothing but lean corded muscle. Energetic, supremely confident and vastly enjoying the adventure. I thought he had to be an alien, but I wanted to be him when I got old.