There is no physical/chemical/biological reason you can’t live indefinitely with the health and vitality of a 25-35 year old. Aging isn’t a law of nature.
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In fact we know how to live forever, control our telomeres. We know it works because cancer exists. We just can’t control it but controlled cancer is effectively immortality.
For the jellyfish, we don’t know. Their cells are indifferentiable by age and they’re bastards to study, with only one scientist in Kyoto having managed to culture them [1].