I recently received an e-newsletter from a respected doctor with the subject line “Growing older is inevitable, aging is optional.” Here’s the thing, actually: Stuff wears out; aging is not optional. Starting in our 30s our normal physiology starts shifting in the direction of entropy, affecting the functioning of our brains (vision, mood, cognition, and sensation), digestive tract, reproductive abilities, the strength of our bones, and more. There is obviously great individual variation both within one individual (my hair is only a bit gray, but my glasses were progressive by the time I was 50) and between individuals (my brother went gray in his 20s!). But gradually, and inexorably, the changes of physiology as we age are well known, and thankfully now measurable and actionable, if we know what we’re doing.Our natural lifespanOpinions abound as to what is our natural lifespan. The Bible might have us counting our lives in centuries, but most modern thinkers opt for somewhere between 75 (“Life really isn’t living past the age of 75,” says Ezekiel Emanuel, one of the architects of Obamacare) and 125 (th …