While The Clone Wars begins as a series about heroism and the good winning over the bad, it’s clear by season 7 (and if you watched Revenge of the Sith in 2005) that this story is actually a tragedy. And nothing about this war is as tragic as the clones themselves. They were genetically engineered for one purpose: to kill and die on the frontlines for their makers. Born into servitude, all most clones ever get to know during their lifetimes is the battlefield.
The end of the Clone Wars didn’t mean freedom for the clone troopers, either. In fact, they were used as the tools of a new galactic order, as the Republic turned into the Empire. Three years after the Grand Army of the Republic first left Kamino, the clones were enlisted as the first generation of Imperial stormtroopers tasked with keeping planets in line and quelling resistance to the new, authoritarian government.
Control chips implanted in their brains at the time of birth also compelled these soldiers to turn on the Jedi during Order 66. The Clone Army became a useful weapon of mass extermination with a single order. Given no other choice but to follow orders, the clones continued to serve Emperor Palpatine until their deaths.
These clones served as Imperial stormtroopers for years until they were eventually replaced by natural-born humans whose loyalty and fanaticism towards the Emperor could be easily exploited. Meanwhile, the cloning facilities on Kamino were shut down and the remaining clones were retired from service.
By the time of the Original Trilogy, it seems that there weren’t any clones left from the Prequel era. But did they all die out before the start of A New Hope? It’s unclear to this day exactly how long a clone is designed to live. We do know that they were engineered to mature twice as fast as humans. The cloners of Kamino did this so that their creations could develop into full-grown adults in half the time it would take normal soldiers. It only took the clones about 10 years to be ready for battle.
Could this mean that clones were also designed to reach the end of their lives twice as fast? It’s possible since clones were created as disposable units for the Republic’s war machine, a quick way to grow an army without having to draft soldiers from the galaxy’s own population. In this way, the clones weren’t all that different from the enemy Droid Army.
If the Clone Wars began in 22 BBY (Before the Battle of Yavin), that would mean that 22 years had passed in between the war and A New Hope. That’s 44 years in terms of a clone’s accelerated growth. Now, taking into account that clones would need 10 additional years to develop (20 in clone years), that would make a member of the very first batch of clones about 64 years old in clone years at the start of A New Hope. This would certainly explain why Rex looks so much older in Star Wars Rebels, which is set almost two decades after the Clone Wars. He’s aging very quickly.
Of course, this doesn’t exactly answer the question of how long clones are meant to live in Star Wars. The closest answer is that, like natural-born humans, a clone’s lifespan varies, although they probably weren’t designed to live more than 50 human years (that would make a clone 100 years old). If the average life expectancy of humans in the galaxy far, far away is similar to our own, it’s about 70 years for men, meaning that clone life expectancy can be halved to just 35 years.
We do know that Rex was part of the strike team that infiltrated the forest moon of Endor in Return of the Jedi, which is set 4 years after A New Hope. That would make Rex about 72 in clone years, the fittest old man on the battlefield. That said, this is a retcon introduced at the end of Rebels, so the same rules may not apply to him.
Still, the fact that no clones appear in the entire Original Trilogy (except Rex and another retconned clone named Crag, the stormtrooper Obi-Wan uses the Jedi mind trick on in A New Hope) probably means that most of them — but not all — were already dead by the time Luke Skywalker went on his grand adventure and defeated the Empire. Dead or alive, they were certainly forgotten by then.