Although it seems that your gut microbiome (which is fully capable of regenerating) will simply never grow old, unfortunately it is not. In the same way that the knees of a person in their 30s do not squeak like a centennial, the gut microbiome changes with age.
In a study comparing the gut microbiome of Northern Italian centenarians with young adults (the 30 Somethings), they found that elders had a loss of overall healthy germs and an increase in inflammatory germs. The genetic composition of the older microbiome was less able to process fiber and produce short-chain fatty acids (SCFA). As you may know, SCFAs are the currency of gut health and are crucial for whole-body health. So being less able to produce them is a sign of poor health or in other words, aging
In a different study outside Ireland researchers have shown that aging is associated with a decrease in gut diversity. Diversity is important for gut health and when we lose diversity, we tend to become vulnerable to disease. And once again, they saw the loss of the microbes that produce SCFA.
Taken together, it seems that the gut microbiome actually decreases with age and that these changes can help to explain the onset of a disease that manifests as we age. For example, a 201