While it would seem that your gut microbiome (which is completely capable of regenerating itself) would simply never age, it is unfortunately not the case. In the same way that a 30-something person’s knees don’t creak like those of a centenarian’s, the gut microbiome changes with age.
In a study comparing the gut microbiome of Northern Italian centenarians to young adults (30-somethings), they found that the elders had a loss of generally healthy microbes and an increase in inflammatory microbes. The genetic makeup of the elder microbiome was less capable of processing fiber and producing short-chain fatty acids (SCFAs). As you might know, SCFAs are the currency of gut health and critical to health throughout the body. So to be less capable of producing them is a sign of diminished health or, in other words, aging.
In a different study out of Ireland, researchers showed that aging was associated with a decline in diversity in the gut. Diversity is important to gut health, and when we lose diversity, we tend to make ourselves vulnerable to disease. And once again, they saw a loss of the microbes that produce SCFAs.
Taken together, it appears that the gut microbiome does, in fact, decline with age and that these changes may help to explain the emergence of disease that occurs as we age. For example, a 2017 study found that brain amyloid, the thing that causes Alzheimer’s disease, was associated with an increased level of pro-inflammatory microbes in cognitively impaired elderly.