There are two prevailing theories of aging.  One says there is inborn genetic program which makes organisms grow old. The natural selection pushes older organisms down to deterioration. However, the same natural selection works by favoring genes involved into reproduction. It turns out that after reproduction ends, genes are beyond the natural selection. Many scientists argue that aging couldn't be genetically programmed.

Another theory states that aging is an inevitable consequence of accumulated toxins; NA-damaging, free-radical molecules, radiation, disease and stress bringing our body to the point that can't rebound. This theory has dominated aging research.

Recently researchers from Stanford University  discovered the alternative model of developmental drift. They found age-related shifts in levels of three transcription factors, that turn genes on and off. These shifts trigger genetic pathways that transform young into old. They found none of the stressors affected this mechanism. So aging is not the result of chemical damage of cells. In fact this is a new way to think about how to slow the aging process.