Why do we die of old age?
We die of old age because the biological cost of making us live forever does not improve our chances of passing on our genes. In nature something else would kill us eventually so there was never a reason to evolve the ability to live indefinitely. There are multicellular organism  that are biologically immortal(all unicellular are), they live slow low activity lives. A lot of the evolution of more "advanced" life is trading regeneration or longevity for higher activity. Literally live fast die young, leave more offspring.