By his adult years, a mysterious "Green Plague" was killing Kryptonians by the thousands, and upon researching the matter, Jor-El discovered that its cause was growing radiation produced by Krypton's increasingly unstable core. This process was going to cause the planet to explode.
As it turned out, all Kryptonians are born with a genetic bond to Krypton itself, which prevents them from leaving the planet. However, Jor-El was able to successfully remove the bond from his son's DNA, allowing Kal-El to safely travel all the way to Earth.
In The Last Days of Krypton by Kevin J. Anderson, young Kal-El and the people of Argo City (along with refugees from another city living there after General Zod destroyed their home) are the only survivors of Krypton's actual destruction.
Notably, upon learning of Brainiac's modus operandi of destroying planets by destabilizing nearby stars when he is done collecting cities and knowledge from said planets, Superman openly speculates that Brainiac destroyed Krypton, which was destroyed when its sun went nova soon after Brainiac abducted Kandor and Argo.
Due to the success of living in a sustainable atmosphere with little to no complications, a Kryptonian's body can remain in the same physical state for as long as possible. This can enable them to live well into their hundreds to even thousands of years with every detail of their appearance remaining unchanged.
It depends on the version you use as a reference. In DC ONE MILLION, he lived on forever. In Kingdom Come, even though he was already cleary in his 50's or 60's. He lived on for another 1000 years.
Kryptonians were once born naturally, but this idea was soon discarded in the wake of advancements in genetic engineering, and they ultimately decided to control the population by growing fetuses underwater in orbs within Genesis Chambers (monitored by aquabots), which provided them with all they needed to survive ...
Travelling to Krypton's past, Strange sought out Seg-El to warn him of an approaching threat, that someone from the future was planning to destroy Krypton, and if the timeline was changed then Seg-El's grandson, Kal-El, also known as Superman, "the greatest hero in the universe", would never exist.
The Guardians revealed to Jor-El that they already knew of Krypton's impending doom and had chosen not to intervene. In their view, Krypton's destruction was a matter of fate, and not to be circumvented.
Zod appears in Superman: Earth One in which he is called Zod-El, brother to Jor-El and thus Superman's uncle. Zod-El is a Kryptonian soldier who wages a six-month civil war against the Science Council, and the one responsible for Krypton's destruction by striking a deal with the Krypton's warring neighbor Dheronians.
Before Kryptonite poisoning could claim their daughter's life, Zor-El and his wife Allura built a space rocket and sent her to Earth where she was found by her cousin.
Where did Supergirl come from if Krypton was destroyed?
Depends on what Universe, but her general history is that she is the cousin of Kal El. In the original comics, a city of krypton survives the planets destruction. However, the residents of the city slowly die of radiation poisoning. Zor El sends his daughter to earth to be raised by her cousin.
Kryptonians don't possess any powers on their home planet Krypton (although sometimes it was stated that they were capable of flying but lost this power due to overrelliance on their advanced technology), due to its red sun and natural higher gravity.
However, after Krypton was destroyed, pieces of Superman's homeworld were exposed to radiation in the blast and became the dangerous compound known as Kryptonite. When Superman is exposed to Kryptonite he becomes as vulnerable as any human on Earth.
Since the destruction of Krypton, Kryptonians have become nearly extinct. It was originally believed that Superman was the sole survivor of Krypton, but since then many additional Kryptonians have appeared.
As awful as it sounds, Zod's plan is likely to have worked. By forcefully rationing more resources, the Kryptonian elites could have created Jor-El's device in time to at least postpone Krypton's destruction, if not curtail it entirely.
Depictions of Superman's homeworld of Krypton have varied wildly over the years, and in one version, Kryptonians were kept from colonizing the rest of the galaxy thanks to a genetic alteration made to the species by a being called the Eradicator, a being who would go on to a play a role in Superman's death and rebirth.
This Green Lantern worked tirelessly to try and save the planet, but every DC fan knows his efforts ultimately fail. The Green Lantern was worked so hard trying to save the planet that he eventually blinded himself and Krypton explodes because he can't get the materials to stabilize the planet in time.
The reasons are unknown, be it due to a solar storm altering his mind, Darkseid, Mister Mind, some new form of Kryptonite developed by Lex Luthor, or that he was simply always evil, but regardless, he went on a killing spree as his friends tried to find out what was wrong, to reverse it.
Krypton is Superman's planet of origin. The inhabitants are called Kryptonians, and they are physically equal to humans. The planet was destroyed accidentally due to it's core running out of energy, and exploding in the process, destroying the planet. The planet is around 8.7 billion years old.
This means that once exposed to a yellow sun, Kryptonians will age normally up to their prime, but the aging process will slow down considerably when their bodies reach a peak level of maturity. They are not immortal per se, but they age so slowly that they may as well be.
For one, the Kryptonians don't have a yellow sun, therefore they don't even have powers. So on a physical level, they are no match, as “regular” Asgardians are above super-soldiers in strength, speed, stamina, etc.
