This is a very complex question. Turtles and tortoises have been on planet Earth even before the dinosaurs. They are that old. The difference between a turtle and tortoise is that the turtle stays in water while the tortoise stays on land. But both creatures have one thing in common: they live to a ripe old age, from 120 years to almost 200 years!

There's an evolutionary answer and a biological answer according to livescience.com. Animals such as snakes and raccoons love to eat turtle eggs. To pass on their genes, turtles have to live a long time and breed frequently, sometimes multiple times per year — and lay a lot of eggs. It is kind of amazing that the world is not overrun by turtles, given how many offspring they have.

The biological mechanism behind turtles' longevity is more complicated. One clue to turtles' longevity lies in their telomeres, structures composed of noncoding strands of DNA that cap the ends of chromosomes, Neuman-Lee said. These structures help protect the chromosomes as cells divide. Over time, telomeres get shorter or degrade, which means they can no longer protect their chromosomes as well, leading to issues with DNA replication. And errors in DNA replication can lead to issues such as tumors and cell death.

Scientists know longevity increases as metabolic rate drops, there is still some controversy as to why. Many believe that ageing is linked to the production of free radicals, reactive particles that are released into the body as it burns fuel. There's plenty of evidence to suggest that free radicals damage proteins and DNA

Tortoises and turtles in general have slower metabolism. They grow at a steady rate and hibernate. Tortoises, like other reptiles, are cold blooded, or ectothermic, so they need to ‘warm up’ by absorbing heat from their surroundings to keep active. In winter, when food is scarce and the temperature falls, tortoises in the wild stop eating, their breathing and heartbeat both slow and they go into a deep sleep. The important parts of their body tend to age slower compared to that of humans and birds. They grow very slowly, which makes them survive without food and water even in harsh conditions. For most animals, there are three basic ways they can die: disease, injury, or old age, which is also called senescence. Tortoises are the most famous negligibly senescent animals.

Source: livescience.com, acs.org