I have heard, on many different occasions, the idea expressed that humans "ought" to live to be 120 years old. Frankly, when I look at this figure I am moved quickly to the basic conclusion that 120 is just a nice rounded number that lies just barely beyond the final ages of our average centenarian. I will say that if there is real, hard evidence that a human being should not, in so-called "proper" conditions, live to less than 120 years, I have never heard of it. I have certainly heard many wild and vastly unsubstantiated ideas about it, though.
Ultimately, the number 120 is almost a given. Somebody asks, "How long can a man potentially live?" Somebody else thinks of the oldest person they heard of...a 103 year old. Another person has heard of a 112 year old. These people are the oldest we know of, so we use them as our model. From there, arriving at 120 is a mere matter of assuming that SOMETHING, SOMEWHERE along the way hurt their health in a permanent fashion. We add a few years to compensate for the misfortune and we arrive at a nice, round 120. Not too difficult to come up with off the top of one's head, really. This is a simple common sense deduction. We draw the theoretical line at around 120 years.
As far as 120 years being the amount of time we were "designed" to live, and which is out of reach based upon our "environment and personal habits", I would say that this is a somewhat dubious conclusion. An entire 120 years? Really?
First of all, there has been, at best, only a relative handful of people amongst a historical ocean of billions upons billions of humans that have lived to 120 years. So, it is most logical to assume that 120 years of life probably has much more to do with luck than with any course of life chosen by an individual. Perfectly healthy people -people healthy in body AND mind- still die at 60 or 70 or 80 or 90. There is no reason to believe they ought to be living to 120, anymore than there is reason to believe that they ought to have brown eyes.
Secondly, people have a tendency of interpreting religious texts very literally. Many examples can be found in various texts of antiquity, from philosophical to religious, explaining that man does not live an ideal life, and therefore his years are comparably substandard in number. Most of these texts, though, are referring not to the so-called "physical" world or the equally ambiguous "metaphysical" world. They are descriptions of states of consciousness...of conditions of the mind. The man that lives 120 years according to these texts, is a man that lives a remarkably complete life in a metaphorical sense, because in whatever number of years his physical body expires, he has lived as if he lived forever. In this case, the age of 120 is a symbolic expression of a life so contented that it is, symbolically speaking, "complete" and "whole"...lacking in nothing. No ACTUAL correspondence between this ideal and the physical world is necessarily part of the bargain.
After all, few of the sages and philosophers that wrote about such things had any hope of living that long themselves! For as much as people say "we don't live long because we don't live right", the fact of the matter is that the average human lifespan on this planet has gone nowhere but up since the times of our prehistoric ancestors. Ancient tribal humans certainly did not entertain the idea of living on for ages and ages, as they were quite contented to live long enough to have a family at the ripe age of 18 to 20. Even a mere 500 years ago, the average lifespan in most places was holding steady at somewhere between 30 and 60, or thereabouts.
In such times of human existence, the common sense deduction that one might live to 120 which we draw today would've seemed like pure fantasy! After all, how many examples of 100-plus year olds do you think existed then? They were so rare that they bordered on unheard of, plainly a work of myth.
I suppose my point is that if humans really were to live to 120 years, then how is it possible that in thousands of generations of life, in every walk of life, we have proven to be failures?! Ultimately, one comes to find that when people are questioned as to exactly how they live to 120 years of age, the answers they receive don't seem to have anything but vague generalizations concerning pseudo-religious beliefs and hack science. A person could starve themselves of all delightful food in favor of special herbs and an incredibly strict diet, but they might still die of a heart attack while munching on their greens and organic apples at the age of 50.
At best, one might say that "a human's biological potential,
in a perfect world, is 120 years." In other words, if we could somehow live a life away from absolutely every danger and unrest of mind or body, we MIGHT live to be 120. Of course, none of us can do that. We even find that those who do, we call oftentimes agoraphobes...and the lifestyle of isolation they lead is usually not interpreted as "enjoyable", and certainly not as "healthy".
To examine a human being outside of his environment is to examine a creature that doesn't really exist. "We'd live to 120 BUT..." This is very misleading. There is, as far as science and civilization have found in the course of their lineages, no such thing as a human without a corresponding environment. Thus, every statement that claims that we live to 120 except for our environment is comparable to saying," If we lived outside of the world, we'd certainly live to 120 years." This isn't really saying much at all, because the edge of the world is liekly a fantasy devised by science-fiction writers. Not even physics or astronomy lays claim to discovering "the edge of the universe".
So, for the time being, we just have to accept that humans aren't "designed" to live to 120. That is, no more than the Universe is "designed" to circumvent this design. Everyone gets those 120 years in one way or another, but for some it goes by quicker than others.