I could swear there’s a ’50s or ’60s SF writer who came up with the idea of immortal aliens regenerating by shifting to an infantile stage. I don’t think it was Niven, any thoughts?
Anyway, here’s a real-life immortal jellyfish that does that. This moves immortality from the level of Henrietta Lacks’ cells to full-blown animalia.
Technical application to humanity left as an exercise for whoever wants to get rich fast and live forever.