Well, once we discover a way to prevent or reverse aging, everything else would be a matter of time, of which you would now have an effectively unlimited amount.
For example while dogs have a much shorter lifespan than humans they seem to face all the same age-related declines humans face. This holds for animals close to or exceeding our mass as well. If different parts of the body wore out simply due to e.g. mechanical stress, you would expect e.g. dog joints to be perfectly healthy after 15 years since they are biologically very similar to human joints, when in fact dogs tend to still have arthritic and otherwise fragile joints at 15 (or earlier).
Everything declining together, just on different timetables in different animals, seems to suggest that aging is controlled by either a single mechanism or some interwoven set of mechanisms that cause everything to fail at the same time, rather than individual parts failing because "they wear out." If you could target that mechanism(s) then maybe you can make humans live as long as e.g. tortoises. But of course we haven't confirmed that such things exist, but the above is at least some motivation for why they might exist.
Without a coherent theory of senescence this is all conjecture, but there's compelling evidence that there's some set of hidden variables we aren't seeing that is "pulling the strings" when it comes to aging.
Let's just think logically about it for a bit. Your cells come with all kinds of repair machinery and redundant DNA encoding. All cells do. But there are single-celled creatures that don't age and reproduce by cell division, and they haven't lost their function despite billions of years and millions of generations. Same for humans and whoever else reproduces sexually — there's A LOT that needs to go just right for a new human to be born, including making billions of copies of parents' DNA, yet it does work just fine most of the time. This tells us that the DNA copying process must be very robust, and that with age, it's these mechanisms that keep everything in check that deliberately get weaker or stop working altogether.
Recent research proves it.
For example, there's evidence that organisms propagate their age throughout all tissues using some kind of signaling in the bloodstream: https://www.nature.com/articles/ncomms13363
Even more bizarre, the aging clock, as determined by DNA methylation, gets reset to zero at the beginning of embryonal development: https://hplus.club/blog/how-and-when-mother-nature-resets-th...
There are definitely a number of processes involved in aging that are pre-programmed as you suggest, namely the hormonal clocks involved in puberty and maturation, and then later senescence.
It's not particularly useful to compare humans/mammals to single-celled organisms, which operate under way fewer constraints. They don't have to deal with cancer, as just one very basic example. They also don't have to figure out how to clear old junk from their tissues and individual cells. They just start over with a clean slate every time.
Signals in the bloodstream - various hormones and immune cells - are obviously involved in aging. But there are many of them, and we are just starting to understand their different interactions. There is no doubt that aging research is going to yield ways to suppress or promote the action of certain hormones, replenish/reactivate certain stem cell populations, etc. but better understanding of intercellular regulatory networks is needed for that.
To call DNA methylation the determinant of an "aging clock" is also too narrow or simplistic. DNA methylation is the mechanism of cell differentiation, but it's "metadata" that has to be copied over between cells when they divide, just like the DNA itself. Early in embryogenesis, the DNA methylation pattern is reset and overwritten using the regulatory cascade of master control genes (homeobox, etc.) - so it is expected that the methylation pattern will have lower fidelity with each division cycle before or after that point.
This is the golden age of biology - we have just gotten a hold of the tools that will let us solve these problems - but it's messy and complicated. Investing in your understanding of these processes can have a huge payoff - I recommend reading some of the Nature Reviews articles like https://www.nature.com/nrg/ - I've always found them to be incredibly helpful in summarizing the state of the art.
While that makes it really unlikely that aging can be reversed, it certainly isn’t logically impossible. All you need to do is expell the chaos, and get one good quality regeneration (possibly taking about 7 years) and then you’d be right back to 18 years old again. That might make it worthwhile enough to be willing to have your body burned by a billion laser shots and float in a vat of liquid for 7 years. We might get to find out in our lifetimes? who knows.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4712935/
Also consider that long life may not produce the advancements you might hope for. There's a saying: "Science progresses one funeral at a time." It's a reference to the fact that individuals typically hold fast to their ideas-- scientists included. Certainly we may react to & integrate new knowledge, but at some fundamental level we are hindered by the cognitive foundations of knowledge laid down when we're younger that are not automatically escaped on new information. That's not even accounting for pride & ego causing people to cling even more fiercely to their own ideas.
You might think that on ongoing infusion of new talent would prevent this, but massively extended age would also entrench the older generations firmly into the power structures of the scientific community, creating at least a partial gerontocracy. Heck we have a bit of that now: Plenty of PhD candidates get stuck "playing the game" and not bucking the status quo too much, only guided down paths their advisors find acceptable. If you have something that significantly contradicts the current consensus, then without a very good & open minded advisor willing to go to bat for you, you might as well shelve it, do something mundane & acceptable for your dissertation, and dust off that other idea when you're done. This situation would not improve with the further ossification of power structures.
If we got there though-- massively extended lifespans, the gerontocracy aspect of things in society in general would be also be a difficult sociological issue. With a 300 year lifespan, by the time you got to 200 you would probably consider anyone under the age of 50 as little more than children. The issue of wealth inequality would be incredible as generational wealth accumulated by anyone remotely careful with their finances would, by late-middle age, dwarf the resources of anyone starting out.
There could be some interesting & positive aspects to all of that though too. Imagine have half a century to explore intellectually or otherwise before you were expect to settle into a path in life. And if we found societal of medical methods to help people break out of psychological & intellectual inertia then a person might expect to have multiple very different careers in their lifetime.
Anyway, all of that would be a very long ways off. At my age I might still hope that leaps in medical advances could give me an extra 10 years of productive healthy life, but probably not much more. And I've also gone a long ways off the original topic, so I'll end it here.
You might be able to restore the body's ability to regenerate itself. But that's a terrible idea. I shudder at the idea of living to 120, never mind 200. Imagine being born in 1821 and having to live today. Imagine how confusing that would be.
Next time someone asks me to do the dishes, I will tell them the same. Hopefully they also forgot about the sun providing energy into the system.
Don't cheat! Don't use the caloric needs of a fully grown adult. Their bodies stoically accept a lot of the damage done by entropy. You have expend additional energy to reverse it.
Don't cheat, remember to account for a population growth rate == the current birth rate.
If you don't like having to account for the growth rate, please give a moral explanation for why some select few get to live forever and others croak.
How much power do you need? How are you supplying this power? It's cute to say "Sun". But solar panels (let's assume 100% efficient) require surface area that would otherwise be used by plant and animals for their existence.
So live forever by
1. Radically transforming the Earth into a massive ball of polycrystalline silicon 2. Giving eternity to only a (very) select few.
We are not closed Systems
The live forever types are just assuming that energy will be plentiful enough to sustain hundreds of billions of people indefinitely [1] and heat easy to dissipate
[1] The population of the earth is stable at 7 billion only because people have this nasty habit of dying. If we stopped doing that, the population would grow exponentially at the birth rate of 1.7%. Or a doubling every 42 years. In 160 years (not that far off from the current record of longevity) the world's population would hit 100 billion people.
Of course the "live forever" types never mention that eternity is a promise for the privileged, and not for the masses.
As long as changes happen gradually, I don't see the problem. If they don't, people of almost all ages will be confused.
So you're in-between the age of 40 to 60? Let's say you're 60, born in 1961:
- Your local pop radio station often plays a rock song from your adolescence. The Queen of Pop whose posters adorned your room and so scandalized your grand-parents still sings at the Super Bowl.
- The Star Wars movie you geeked out as a teen/young adult has become a billion dollar franchise your grandkids drag you to
- The ATM you learned to use thirty years ago are still around basically unchanged.
- the quarter the tooth fairy left you as a kid that you found in the tin can of boyhood treasures is still accepted as currency
Compare that to a 60 year old person born in 1820.
They have yet to have seen a train: most people in the world at the time lived their whole lives in their villages far from any railroad. China's first railroad was built in 1875, and quickly dismantled. (China, alone, in the 19th century had between 30 to 38% of the world population)
Most likely this person lives in feudally arranged society. The emancipation of the serfs in Russia was in the mid 1800s, and that was the last of the European countries to remove feudalism.
In the anglo saxon world, women were effective properties of their spouses.
The superiority/inferiority of races was obvious to the learned classes.
Slavery was practiced throughout the world except Europe and then the US.
Homosexuality was punishable by execution most everwhere in the world
Sexual roles were well defined and strictly obeyed according to local customs (i.e. they weren't universal norms, but norms existed universally).
The only way to fly was in a hot air baloon. The only people who ever witnessed this lived in Paris or in London
Steel existed but was extremely expensive and not used for construction. Wrought iron was used instead, and it was a brittle mess that resulted in catastrophes. As a result the buildings in urban areas were typically no more than 6 stories high. Religious buildings were much taller, but projects that took centuries to build.
Communicating with someone across the Atlantic took weeks.
Now, let's say our hypothetical person living in 1880 and born in 1820 lived another 60 years.
The airplane is invented and used to devastating effect in multiple wars
The automobile is invented, and by 1940 mass produced.
Throughout the world, therefore, buses provide transport to remote area that aren't serviceable by rail
Skyscrapers pop up.
A world war has impacted most people in the world. A new one has just started.
A communist society exists and has mechanized death killing millions.
Telecommunication, first by wired-telegraph, then by wireless telegraph, and finally by radio exists and enables instant communication across the globe. The most remote villages can receive the BBC world service, often in their native language.
Our gentleman most likely wouldn't be aware of this development but classical physics has been shattered, the concept of time turned upside down. The atomic era has also begun, but most people will be blissfully unaware for another 5 years.
Penicillin is about to revolutionize life expectancy.
I think this person born in 1820 would be very uncomfortable if living today.
Unless you're religious. Then it's not death you should fear, but the judgement.