Nope, humans age due to telomeres shortening each time the cell divides. You decay or rot because your body no longer is maintaining homeostasis...blood no longer moves through providing nutrients. oxygen etc, and it allows for forgein bacteria, fungus etc to start growing. Cells divide constantly for various reasons, if they didnt your immune system couldnt adapt, your brain wouldnt retain memories or skills, couldnt create muscle memory etc without what you call decay. Immortal literally means not dying, its in the actual word. Doesnt say unaging, immune to disease, immune to injury. Just means your not gonna die. Not that you'll stay forever young.