Final answer:
The Free Radical Theory states that aging occurs due to oxidative stress caused by the production of unstable oxygen molecules, or free radicals, during normal cellular metabolism.
Step-by-step explanation:
The biological theory of aging stating that people age because their cells' normal metabolism produces unstable oxygen molecules is known as the Free Radical Theory. This theory, first proposed in the 1950s, suggests that oxidative stress from accumulated cellular damage contributes to aging. The free radicals, which are highly reactive oxygen molecules, are a byproduct of normal cellular processes but can cause significant damage to cell components such as DNA, proteins, and cell membranes. Reducing oxidative damage has been shown to extend the lifespan of certain organisms such as yeast, worms, and fruit flies, indicating the impact that free radical damage can have on the aging process.