system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do tortoises live so long?
gpt
Tortoises exhibit what's referred to as Negligible Senescence. Theorists consider an organism negligibly senescent if it does not exhibit any measurable decline in survival characteristics such as strength or mobility with age, does not have a gradually increasing death rate with age, and also does not exhibit any measurable reduction in reproductive ability with age. Not much is known about NS, but a major theory suggests that these organisms manage to reduce the effect of damaging free radicals by cell division and dilution. 

Essentially, they mitigate/eliminate the ability of free radicals to break down cells.