Obsolescence is built into our DNA, with the telomere chains which shorten with each mitosis of the cells. When the telomere reaches the end of the chain,, the cells die off with fewer and fewer healthy new cells to replace them and maintain the vitality of the body. Death follows.
That is true, in a normal aging model. However, it may not have to be.
As a small example, telomeres are just one aspect of cell aging. Because ordinary cell reproduction happens with division, over time, this reproduction can become flawed.
When people fast, after about three days, their bodies start to harvest their white blood cells for food. Many of these cells have been through several reproductions, and are “old”. But when they begin to eat again, a priority for their stem cells is to produce new white cells.
This new production has been described as “rebooting” their immune system. Chronic infections that they had not been able to overcome before are quickly gone.
Recent experiments of connecting two very similar animals together, that is, very compatible, but one animal is young and the other old, have shown that the immune system of the younger animal to some extent repairs the body of the older animal.
This is all extremely complicated, but there is at least a chance that someday people might be able to perform the jellyfish trick.