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Modern biological theories of aging in humans fall into two main categories: programmed and damage or error theories.

The programmed theory has three sub-categories: 1) Programmed Longevity. Aging is the result of a sequential switching on and off of certain genes, with senescence being defined as the time when age-associated deficits are manifested. Dr. Davidovic et al discuss the role of genetic instability in aging and dynamics of the aging process (1). 2) Endocrine Theory. Biological clocks act through hormones to control the pace of aging. Recent studies confirm that aging is hormonally regulated and that the evolutionarily conserved insulin/IGF-1 signaling (IIS) pathway plays a key role in the hormonal regulation of aging. Dr. van Heemst discusses the potential mechanism underlying IIS and aging process(2). 3) Immunological Theory. The immune system is programmed to decline over time, which leads to an increased vulnerability to infectious disease and thus aging and death. It is well documented that the effectiveness of the immune system peaks at puberty and gradually declines thereafter with advance in age.

The damage or error theory include 1) Wear and tear theory. Cells and tissues have vital parts that wear out resulting in aging. Like components of an aging car, parts of the body eventually wear out from repeated use, killing them and then the body. So the wear and tear theory of aging was first introduced by Dr. August Weismann, a German biologist, in 1882, it sounds perfectly reasonable to many people even today, because this is what happens to most familiar things around them. 2) Rate of living theory. The greater an organism’s rate of oxygen basal metabolism, the shorter its life span (5). The rate-of-living theory of aging while helpful is not completely adequate in explaining the maximum life span (6).Dr. Rollo proposes a modified version of Pearl’s rate of living theory emphasizing the hard-wired antagonism of growth (TOR) and stress resistance (FOXO) (7). 3) Cross-linking theory. The cross-linking theory of aging was proposed by Johan Bjorksten in 1942 (8). According to this theory, an accumulation of cross-linked proteins damages cells and tissues, slowing down bodily processes resulting in aging. Recent studies show that cross-linking reactions are involved in the age related changes in the studied proteins (9). 4) Free radicals theory. This theory, which was first introduced by Dr. Gerschman in 1954, but was developed by Dr. Denham Harman (10, 11), proposes that superoxide and other free radicals cause damage to the macromolecular components of the cell, giving rise to accumulated damage causing cells, and eventually organs, to stop functioning. 5) Somatic DNA damage theory. DNA damages occur continuously in cells of living organisms. While most of these damages are repaired, some accumulate, as the DNA Polymerases and other repair mechanisms cannot correct defects as fast as they are apparently produced. In particular, there is evidence for DNA damage accumulation in non-dividing cells of mammals. Genetic mutations occur and accumulate with increasing age, causing cells to deteriorate and malfunction. In particular, damage to mitochondrial DNA might lead to mitochondrial dysfunction. Therefore, aging results from damage to the genetic integrity of the body’s cells.

Keby niekto prestal starnúť a začal mladnúť, tak by sme to pokladali za čary. Pritom je to možné, len je to (podľa zákona rastu entrópie) vysoko nepravdepodobné (asi tak nepravdepodobné ako, že na tisíc hodených kockách padne šestka).

Proces starnutia nastáva, pretože zmenené energetické stavy biomolekúl spôsobujú, že sú neaktívne alebo nefunkčné. Identické udalosti sa vyskytujú aj predtým, ako sa objaví fenotyp starnutia, ale procesy opravy a výmeny sú schopné udržiavať rovnováhu v prospech fungujúcich molekúl; inak by druh zmizol. Po reprodukčnom dozrievaní sa táto rovnováha pomaly posúva k takej, pri ktorej je menšia pravdepodobnosť výmeny alebo opravy molekúl, ktoré stratia svoje biologicky aktívne energetické stavy.

Entropia je tendencia koncentrovanej energie k disperzii, ak jej nie je bránené, bez ohľadu na to, či je systém otvorený alebo uzavretý. Zábranou entropických zmien je relatívna sila chemických väzieb.

Prevencia voči rozbitiu chemickej väzby.. je pre život nevyhnutná...Rozptyl energie môže mať za následok biologicky neaktívnu alebo nesprávne fungujúcu molekulu.