Today's oldster is no older than his counterpart of 100, 1,000, or 10,000 years ago. Though mortality rates have decreased, the process of aging has not been retarded.
By definition "age" is a function of time. Yet, as used now, the term "aging" is not synonymous with getting older. We do not refer to the growth of a child as aging. Implied in the term is a decrease in viability, as well as a loss of information from the senses and hence of adaptation to life. A process that can be observed at various levels, senescence is most obvious in the organism viewed as a whole, less so in the individual organ system, and least at the level of the cell. Yet, it is the cell that holds the key to aging as to many other vital processes.
At least three manifestations of aging occur as changes in cellular activity: