Consider the diversity of life on earth. Every living thing alive today is part of a continuous chain of life stretching back millions of years. The default mode for living things is indefinite life.

Consider that we are used to lifespans because we are complex organisms that have a lifespan, as do most other large complex organisms like us.

Consider the huge variability in aging.

Some species of organisms live only for weeks, while others live for many years (about 100 for most humans, over 1000 for some trees).
There are records of much longer lived humans.

Consider that most smaller organisms (bacteria) do not age - they go on indefinitely.

Two mechanisms have probably led to aging in complex organisms.

Tom Kirkwood proposed the "disposable soma" theory, which states that it is expensive (in metabolic terms) to fix the damage that occurs to DNA (the stuff that carries our genetic blueprint). Organisms that create complex bodies gain an advantage by turning off these repair mechanisms for the non-reproductive cells (somatic cells) and using the energy (food) saved for other purposes (reproduction). Thus, over time, errors accumulate, and eventually the body fails (ages), through slow accumulation of errors.

The other mechanism proposes that there is a selective advantage to populations in which the individuals ages, because these populations have greater genetic diversity, and can more quickly respond to changing environmental conditions. A gene for aging in such a "gene pool" (population) would cause the death of individuals in the pool, but the pool as a whole would be better able to survive in rapidly changing environments than other "pools" without an aging mechanism.
(It is sometimes difficult to remember that in organisms with sexual reproduction, evolution works at the level of a "pool" of genes, and that individuals are only specific examples of possible combinations of the genes in that "pool" {population}.
[Without the mixing of genes by sexual reproduction evolution is a much slower process.]
Individual survival controls the frequencies of genes in the pool, but only at the level of the average over the entire pool. Thus a gene that causes the death of an individual can survive and prosper, if the value of that death to the rest of the pool is higher than the cost of the life lost, and the frequency of death before reproduction is low enough [hence a tendency towards altruism in "old age"].)

Thus populations in which the individuals "age" evolve more quickly, and all complex organisms have some sort of aging mechanism (they wouldn't be complex if they didn't).

These two mechanisms are entirely compatible.

If, as appears likely, the "disposable soma" turned off DNA repair mechanism is the primary cause of aging, then to arrest aging we need only find a way to reactivate the "turned off" repair mechanisms in somatic cells. This would have the effect of increasing our metabolic rate, and allowing people to loose weight without eating less (should appeal to most western nationals).

Reversal of aging will be a more complex process, and may take many decades to develop.

Simply halting the aging process, by activating the repair mechanisms that have been blocked, ought to be much easier, and may be discovered any day, if it hasn't been already.

One more thing to consider: Cloning.
Consider that the cloning process removes the neucleus from an adult cell, and puts it in the cytoplasm of an egg.
Whatever "age" that nucleus was, it now behaves as if it is young, and controls growth accordingly.
Consider that - by the time an egg cell is mature, whatever the cues are that control the aging mechanism, they are resident in the cytoplasm by that stage. They are definitely not resident in the nucleus.

Consider that this cell will go on to grow and that some of it's great .... great-granddaughter cells will themselves become future egg cells.
There is a "reset button" on the "cytoplasmic aging clock" somewhere in the process of creating egg cells.

Considering how "gross" the process of removing a nucleus from one cell and transplanting it to another cell is, it is amazing that cloning works at all.