After correcting for factors other than stress, the scientists examined the peripheral blood mononuclear cells of the ladies, part of their immune system. They found that in the women who experienced the highest stress levels, “the cells had shorter telomeres — bits of DNA that cap the ends of chromosomes. In lab experiments, scientists have shown that telomeres get a bit smaller every time a cell divides, and that when telomeres are worn out, cells can't divide anymore and ultimately die. In humans, older people tend to have shorter telomeres — and by this measure, the most stressed women in the study had cells that looked 10 years older than their chronological age.”
The most stressed women also had lower levels of telomerase, an enzyme that repairs damaged telomeres. Again, reduced telomerase isn't necessarily the key to premature aging, but people with a rare genetic condition that reduces their telomerase production tend to show outward signs of premature aging and often die young of heart disease and weakened infection resistance.
Finally, the stressed women's cells had higher levels of free radicals, a type of highly reactive molecule that can damage DNA. One might argue that women whose children were born with those disorders already had something wrong with their DNA and that stress wasn't the cause. But that wouldn't explain another crucial fact: the degree of cellular damage was highest in women who had been caring for a disabled child the longest.
Stress affects not just ageing but also memory. Robert Sapolsky writes in the Scientific American Mind that “a little stress sharpens memory. But after prolonged stress, the mental picture isn’t pretty.” It’s a wonderful piece, adapted from his book Why Zebras Don’t Get Ulcers, but the link I gave you has just a preview, and the full article isn’t available online. Bummer. Never mind, forget about it