Final answer:
The Free Radical Theory of aging posits that aging occurs due to oxidative stress caused by reactive oxygen species damaging cellular components like DNA. This eventually leads to either the physiological aspects of aging or age-related diseases, with the latter concept being more widely accepted.
Step-by-step explanation:
The theory that states people age because inside their cells normal metabolism produces unstable oxygen molecules that ricochet around the cells, damaging DNA and other cellular structures, is the Free Radical Theory of aging. This theory suggests that oxidative stress from accumulated damage caused by reactive oxygen species (ROS) contributes to the physiological and anatomical effects of aging. There are two interpretations of this theory - one that implies the aging process is a result of oxidative damage, and another suggesting that oxidative damage leads to age-related disease and disorders. The latter is more widely accepted.
It is believed that reducing oxidative damage can extend lifespan in various organisms. However, evidence also shows that manipulations such as calorie-restriction, which are thought to reduce oxidative stress, do not necessarily lead to lifespan extension in all species. Besides ROS, aging is affected by ineffective DNA repair mechanisms, telomere shortening, and changes in cell membranes impacting nutrient transport and waste removal.