The user can regenerate the length of their telomeres. Telomeres are a region of repetitive nucleotide sequences at each end of a chromatid, protecting the DNA fragments; they grow shorter as cell division occurs, an indication of the person getting older. By regenerating the lengths of telomeres, one can extend their own lifespan indefinitely.
Thanks to his Body Supremacy, Stanton Parish (Alphas) has perfect cellular regeneration, repeatedly rejuvenating his telomeres and maintaining his youth for 200 years.
In addition to granting him vast regenerative powers, Tenzen Yakushiji's (Basilisk) symbiote also "eats" away the ravages of time, rejuvenating his telomeres and maintaining his youth for over 200 years.
In addition to healing the sick and being able to resurrect the dead, the Lazarus Pits (DC Comics) also rejuvenate the telomeres of whoever bathes in them. Thus, by continuously using the pits, Ra's al Ghul has maintained his age at its prime for over 600 years.
By continuously lengthening her telomeres, Tomiko Asahina (From the New World) has not only remained young and beautiful, but has lived for over 267 years.
Adam Monroe's (Heroes) cells have regenerated so many times that they are now regenerating and dying in equilibrium, constantly rejuvenating his telomeres and rendering him young for over 400 years.
Wolverine (Marvel Comics) possesses regenerative powers of such levels that they even re-lengthen his telomeres, maintaining his age at his prime.
Mystique (Marvel Comics) rejuvenates her cells whenever she metamorphoses, thus rejuvenating her telomeres, maintaining her age at her prime.
The mitochondria in Aya Brea's (Parasite Eve) body constantly rejuvenate her telomeres, making her appear to be in her early twenties despite actually being in her late thirties.