IIRC, in one chapter of "Lois and Clark", he said to Lois "no one knows how much he has", which means that not even Superman lives forever.
Biochemical changes (as free radicals) in cells over time may play an important role in senescence, but the essence of those changes remains to be determined. Aging is just an accumulation of complex side effects piled on top of each other in a somewhat random uncontrolled way.
The cellular structure of a Kryptonian is similar to that of all humanoids. However, a certain chemical makeup in the ribosomes is capable of absorbing solar energy and storing it. Once the ribosomes achieve saturation levels of this radiation, they begin to re-radiate the energy in the form of a bioelectric forcefield.
Because Kryopton's gravitational field is 50-100 stronger than Earth's, the kryptonian cells are much stronger and denser than a human's. In addition, the bioelectric field which surrounds the cells, makes them thousands of times tougher. The Kryptonian mitochondria absorb certain wavelengths of the electromagnetic radiation emitted by solar fusion. Even under a red sun, this yields increased abilities, which are multiplied a thousand-fold by a yellow sun. The solar energy supplements respiration, such that when cellular materials (perhaps Kryptonian ATP) combine with glucose, they produce abilities beyond those of humans under a yellow sun.
Due to the highly charged nature of his solar storage systems, his immune system deals with most normal poisons and diseases quickly and with no ill effect. Radiation and similar harmful energies are similarly neutralized.
The result is thousands of times longer a life-span for Superman.
Krypton is 50 light years away from earth. If Clark's starship traveled, let's say, 1/10,000 the speed of light (that's still 2.6 faster than any rocket-propelled spacecraft today), then there was no time dilation (process slowdown) effect. This implies that Clark ages at least 120,000 times slower than any human.
Btw Wolverine (from X-Men) is also believed to be of an advanced age (about 150 yrs old) and still looks like he's in his mid 30s. Superman's aging process should be much slower, if there's any.
In Crisis of Infinite Earths, our Superman was confronted with another Superman who apparently was much aged and who proclaimed a great marriage with Lois. There was no indication of outliving her and having birthday parties with his great great grandchildren. Of course, his aging factor can pretty much be speculative, but I don't think it's far too different than our own (exempting times in other dimensions and what not).
Gender: Male Location: Somewhere in The Great Britmerica.
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That was a totally different Superman. Considering he's probably millions of times stronger, faster, and durable than the average person, he sould also age much, MUCH, much, slower--if at all. And since his powers are solar derived, he should be able to exist as long as there are yellow stars in existence.
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Kryptonian cells could be immortal under a yellow sun. In fact some rudimentary organisms are biologically immortal. Germ cells, embryonic stem cells and cancer cells are also immortal.
Gender: Male Location: somewhere looking for trouble
under the right conditions superman would live a long long time and age very slowly...but him being immortal...not likely..."long living" would be a better choice of words.
I think superman would have abused his powers if he were real. I don't believe that a person with that much power ever had any sense to hold back. Even your local manager at wallmart can be a tyrant after one day of being hired. I can see him getting worse with age, and bored too. Why protect a planet that can't get along with each other no matter how many time you save their ass, when you can rule them with an iron fist, take their women, and make them all your lois lanes. Why not find other planets with an even bigger yellow sun, maybe a planet full of being who are a little more forgiving when you fail to save ten cities in one day. What's with all the unecessary pressure buddy? Your a living god! Settle some scores! Crack some skulls! Get laid brother!