Anyone in engineering or technical professions will be familiar with this concept. In brief, it states that the likelihood of failure for a given thingummy will be high when it is new, will see a fairly rapid drop as it's sorted out (because, after all, you cannot test for everything, and the real world has a habit of finding things you didn't test for), and then will stay pretty low until the end of its life. Basically, your stuff will fail when it's brand spanking new, or will fail when it's "old" (and we must bear in mind that for many things, "old" is defined as "one day after the expiration of the manufacturer's warranty"), but in between that, is highly likely to work right without bothering you. If it survives the initial period, it's good to go for a long time.
This applies to humans as well. There are many ways to become broken in childhood, and many diseases and infirmities to which the elderly are prey, but in between, provided one makes reasonable efforts, the first world is a remarkably non-fatal place. I was reminded of this by realising that I'm actually considered a very good risk in health insurance terms; I'm "too young" for such things as prostate trouble, while being old enough to know better than to do things like jumping off buildings, and as long as I keep my weight reasonable (which I have been doing; I did gain a chunk after moving, but have now lost it again and so far am keeping it off, although I could stand to lose more) I'm quite alright regarding many potential issues. My teeth are in maintenance mode; with my paucity of fillings, there's little to go wrong as long as I clean them daily, which I do (in fact, I can't sleep these days without the hint of mint...). My eye exams happen every two years; in between, my vision doesn't change significantly (although at my level of myopia, a quarter dioptre is insignificant; must ask, next time, for a list of places which offer laser surgery).
Sadly, these salad days will inevitably come to an end, and the time will come when I can no longer maintain rude health by means of telling my doctor to eff off. Until that happens, though, I have my health, and intend to enjoy it.