The turtles and its family are some of the longest living species member of the reptiles. There isn’t really a true fact on why they do, but many theories have suggest that their slow metabolism plays a part in it, coupled with a really healthy lifestyle and the ability to survive most predators with its solid protective shell.
With such a long lifespan, they could be growing at a much slower rate than most animals, aging slowly throughout their long life. Since they do not really need to “grow” as fast, their slower rate of metabolism helps them further survive long periods without food or water, giving them an edge in surviving harsh living conditions. This immunity to aging is their main form of defense in order to be able to ensure their own species survival.
Now, with a longer life, they could produce at a much later age or just at a much more irregular rate, they enjoy the luxury of taking their time to reproduce. Interestingly, this genetic trait is found in lobsters and rockfish too. However, this evolutionary advantage comes with a downside as well. Since they age slower, they take longer to develop their hard shells that would protect them, so young turtles become easy prey for many animals. In the case of sea turtles, the only way they could counteract this is through laying a lot of eggs at once, so more of the turtles would made it to the sea before getting picked off by birds and crabs. The turtles would either die young, or really old. Now once they reach a decent age where their shell would be hard enough, they would continue on to live a stress-free and calm life, which does help them to survive so much longer than possible.
So in a word, why do turtles live so long? It attributes to it being an evolutionary defense mechanism, which allows them to thrive as slow, bulky creatures.