Hydra's body is maintained by three lineages of continuously dividing stem cells: ectodermal epithelial, endodermal epithelial, and interstitial cells that generate nerve cells, stinging cells and gametes. Cells are steadily displaced towards the tentacles and the basal disc, where they are shed, so the animal replaces its whole body over a period of weeks.
That permanent turnover is why regeneration is routine rather than exceptional. A cut fragment re-establishes a head organiser at the wound and rebuilds the missing polarity, and the classic dissociation experiments showed cells stripped into a suspension re-forming into a whole animal after aggregation. This is the property that gave the genus its name, after the many-headed monster of Greek myth.
Hydra also shows no clear signs of ageing under laboratory conditions. Studies tracking cohorts over years have found no detectable rise in mortality with age in some strains, which puts hydra alongside a small set of organisms with apparently negligible senescence. That is not immortality: laboratory hydra still die from injury, starvation and infection, and the sexually reproducing forms show a different pattern.