So, how does medical science determine a person's age without looking at documentation? The answer is that they have done it through the study of cadavers over many decades, especially since the practice has been legalized and proliferated in the West. They would look at a cadaver, then at the paperwork like birth certificate, they would look at signs of illness and disorders, and pin everything else not covered by illness or "atypical" lifestyles to the person's actual age. Once they had enough examples with similar signs of aging, they would write medical books filled with their scientific observations and those books would become a point of reference for medical professionals to determine someone's age without looking at birth certificates and such. This is how we got expressions like "You've got the lungs of a 30 year old" or "You've got the liver of a 70 year old".
Now let's take a step back and consider exactly what this means to us in practical terms. First, medical manuals that lay out what condition your body is in at a certain age are always delayed. A modern manual may be based on cadaver observations from 20-50 years ago. Those cadavers used to be people that lived off of outdated health information and advice and may have consequently been living far less healthy lifestyles than we do now. This means that if our bodies match our age, they match the description of what bodies our age were like decades ago or even before that. So, by definition we need to aim for being younger looking and healthier than that.
The second major practical point to make is that the signs of aging are not pinned to disease and disorders per se. What this means is that signs of a certain age are pinned to long term lifestyle, health and nutrition decisions that are, for the lack of a better term, slowly killing us. We often ignore the seriousness of chronic vitamin and mineral deficiencies, and most people out there are not on any supplements like creatine and glutamine, nor are they trying out life extension solutions like Resveratrol together with NMN. As people with significant, long term deficiencies are the norm, they end up being the standard by which we measure the age of a person's body and individual organs. I don't think that a single comprehensive well-publicized study has been done on people who have successfully eliminated vitamin, mineral and other nutrient deficiencies for almost their entire lives. Once such a study is out, widely publicized and widely cited, it'll hopefully create a new benchmark of what a body is "supposed" to look like at a certain age. I believe this would have a positive impact on our entire society. Doctors would give much better advice for your overall health because higher standards would become the norm. Schools would teach kids better health and nutrition information and help foster a better culture of health overall. Finally, preventative medicine would start at home with informed and more carefully planned meals, groceries and so on; this would save us a fortune in medical care and expenses over time, and would decrease the number of debilitating conditions in old age, thereby transforming the retirement home experience completely, as well as the role seniors have in our society.
I don't know if you are as passionate as I am about health and well-being, and I know there are many different situations and life circumstances out there. However, ask yourself a few questions. Are you aware of the lifelong role you have in engineering your old age? Is 70 or 80, in this day and age, too early to die of old age? What does it mean to die of old age, really? Food for thought :)