There are different ways to define biological aging, which often leads to confusion. Basically, aging is a series of side effects of the normal functioning of the human body. We age for the same reason a car ages: the operation (in our case metabolism) does not run smoothly, so damage accumulates in the structure of the body. At some point, the amount of molecular and cellular damage becomes so great that the body can no longer tolerate it and we eventually get the diseases of old age.
These include, for example, cancer (the vast majority of types), Alzheimer’s, Parkinson’s, macular degeneration, osteoporosis and many more. Currently, the probability of becoming ill and dying increases exponentially with age from about 25 onwards.