By the time of the Original Trilogy it seems that there weren’t any clones 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 the clones 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 somewhat disposable units for the Republic’s war machine, a quick way to grow an army without having to draft soldiers from its 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 looked 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).
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 retcon introduced at the end of Rebels, so this is not exactly hard science.