Ageing is not necessary or inevitable.

Negligible senescence refers to the lack of symptoms of ageing in a few select organisms. More specifically, negligibly senescent organisms do not have measurable reductions in their reproductive capability with age, or measurable functional decline with age. Death rates in negligibly senescent organisms do not increase with age as they do in senescent organisms. Negligibly senescent organisms have no "post-mitotic" cells; they reduce the effect of damaging free radicals by cell division and dilution.^{[citation needed]} Another related mechanism is that of planarian flatworms, which have “apparently limitless telomere regenerative capacity fueled by a population of highly proliferative adult stem cells.”^[1]

  

Some rare organisms, such as tardigrades, usually have short lifespans, but are able to survive for thousands of years—and, perhaps, indefinitely - if they enter into the state of cryptobiosis, whereby their metabolism is reversibly suspended. It's hypothesized by advocates of cryonics that the human central nervous system can be similarly put into a state of suspended animation shortly before brain death to be revived at a future point in the technological development of humankind when such operation would be possible.

 

Some examples of maximum observed life span of animals thought to be negligibly senescent are:

• Rougheye rockfish (Sebastes aleutianus) — 205 years^[11][12]
• Aldabra Giant Tortoise—255 years
• Lobsters are believed to live 100 or more years.^[13]
• Hydras are observed to be biologically immortal.^[14]
• Sea anemones generally live up to 60–80 years.^[15]
• Freshwater pearl mussel—210–250 years^[16][17]
• Ocean Quahog clam—507 years^[1