87 Essays on Infinite Lifespans   Robert A. Freitas Jr. they all appear possible in theory. By the time our molecular manufacturing  capability  progresses  to  the  degree  necessary to begin building medical nanorobots, probably in the next 10–20 years, we will have good designs for cell repair devices. The net effect of these interventions will be the continuing arrest of all biological aging, along with the reduction of cur- rent biological age to whatever new biological age is deemed desirable  by  the  patient,  severing  forever  the  link  between calendar time and biological health. These interventions may become commonplace, several decades from today. Using annual checkups and cleanouts, and some occasional major repairs, your biological age could be restored once a year to the more or less constant physiological age that you select. I see little reason not to go for optimal youth – though trying to maintain your body at the ideal physiological age of 10 years old might be difficult and undesirable for other reasons. A rollback to the robust physiology of your late teens or early twenties would be easier to maintain and much more fun.  That  would  push  your  Expected  Age  at  Death  up  to around 700–900 calendar years. You might still eventually die of accidental causes, but you’ll live ten times longer than you do now. How far can we go with this? Well, if we can eliminate 99% of all medically preventable conditions that lead to natural death  [19],  your  healthy  life  span,  or  health  span,  should increase to about 1100 years. It may be that you’ll find it hard to coax more than a millennium or two out of your original biological body, because deaths from suicides and accidents have remained stubbornly high for the last 100 years, falling by only one-third during that time. But our final victory over the scourge of natural biological death, which we shall achieve later in this century, should extend the health span of normal human beings by at least ten- or twenty-fold beyond its cur- rent maximum length.