It seems that your intestinal microbiome (which can completely self-regenerate) would never age, but unfortunately this is not the case. Just as a 30-year-old's knee does not creak like a centenarian's, so does the intestinal microbiome with age.
They found that the elders had a loss of generally healthy microbes and an increase in inflammatory microbes. The genetic material of the older microbiome was less able to process fiber and produce short chain fatty acids (SCFAs). As you may know, SCFAs are the key currency for gut health and health throughout the body. A lesser ability to produce it is a sign of diminished health or, in other words, aging.
In another study from Ireland, researchers have shown that aging is associated with a decrease in gut diversity. Diversity is important to gut health, and when we lose diversity, we tend to become susceptible to disease. And they saw again a loss of the microbes that produce SCFAs.
Overall, gut microbiota appear to decrease with age, and these changes may help explain the onset of disease as we age. For example, a 201