Yeah, you are right. Oxygen which is a life giving gas, is needed by all living things to survive. But Oxygen causes Oxidation and free radicals, and body must continously produce Anti-oxidants to undo the damage done by oxidation.
Oxidation and Reduction are both happening in our bodies, but if we overload the body with plenty of oxidants, chances of damage to our cells increases.
Aging is caused when the oxidative stress of our body increases and body cannot get enough anti-oxidants to fight the oxidative stress. fact is, we are all slowly oxidizing/ rusting away. the result is wrinkles, gray hair, weak eyesight, just to name a few symtoms.
Quoting from Anti-Oxidants article.
"The oxidative challenge in biologyFurther information: Oxidative stress
A paradox in metabolism is that while the vast majority of complex life requires oxygen for its existence, oxygen is a highly reactive molecule that damages living organisms by producing reactive oxygen species
Consequently, organisms contain a complex network of antioxidant metabolites
that work together to prevent oxidative damage to cellular components such as DNA
In general, antioxidant systems either prevent these reactive species from being formed, or remove them before they can damage vital components of the cell.
However, since reactive oxygen species do have useful functions in cells, such as redox signaling
, the function of antioxidant systems is not to remove oxidants entirely, but instead to keep them at an optimum level.
The reactive oxygen species produced in cells include hydrogen peroxide (H2O2), hypochlorous acid (HOCl), and free radicals such as the hydroxyl radical (·OH) and the superoxide anion (O2−). The hydroxyl radical is particularly unstable and will react rapidly and non-specifically with most biological molecules. This species is produced from hydrogen peroxide in metal-catalyzed redox reactions such as the Fenton reaction. These oxidants can damage cells by starting chemical chain reactions such as lipid peroxidation, or by oxidizing DNA or proteins. Damage to DNA can cause mutations and possibly cancer, if not reversed by DNA repair mechanisms, while damage to proteins causes enzyme inhibition, denaturation and protein degradation.["