It is probably true that we don't have fossil evidence of aging from over a billion years ago. Aging probably was 'invented' together with sexual reproduction at about 1200 MYA, but to be on the safe side let's assume that aging certainly existed several hundred million years before mankind.

However, science knows mechanisms of aging - one of them is a cell reproduction limit timed by the length of telomers. Thus aging is a genetic effect and unrelated to access to a "tree of life". And even if we assume that such a tree exists and eating from it can prevent aging, this still does not answer the question why creatures were created with built-in decay in the first place, long before mankind.