Oxidative stress is the unassuming name for the state in which reactive oxygen species - the free radicals of the free radical theory of aging - are present in sufficient numbers to overwhelm the body's antioxidant defenses and thus cause potentially lasting damage and degredation by reacting with important molecules in cellular machinery.
If you care to head back in the archives a way, I explain some of the mechanisms in more detail in a post on the mitochondrial free radical theory of aging: where it is these reactive oxygen species come from, and how it is that they can do so much harm, leading to conditions such as atherosclerosis.
A paper on the rise of oxidative stress with aging caught my eye today:
Oxidative stress has been reported to increase with aging; however, the scientific evidence is controversial. We therefore aimed to analyze the relationship between aging and some markers of oxidative stress.
there was no age-related change in oxidative stress markers in subjects of < 60 years. These findings suggest that age of > 60 years may be associated with increased oxidative stress.
The full PDF version of that paper is freely available, if you are so inclined. As the evidence suggestions, age-related decline is far from a straight line; it seems to speed up greatly after 60, for example, becoming more dependent on the quirks of your biochemistry:
there was almost no genetic influence on age of death before 60
A number of age-related conditions start to emerge at 60, on average, once you start looking at that as a dividing line of sorts:
It may be that once you go beyond 60, which is the age when macular degeneration typically starts developing, the pigment is depleted for several reasons, including increased oxidative stress and a poor diet, both associated with an increase in age.
All the more reason to devote more resources to viable research initiatives aimed at removing sources of reactive oxygen species, or aimed at greatly reducing their impact on our biochemistry. There are groups out there working on the problem, and it's a tragedy in the making that very few of them are funded to the hilt.