http://www.sciencemag.org/news/2019/01/bacteria-your-gut-may-reveal-your-true-age

Bacteroides are the most common bacteria species found in the human intestinal tract.

The billions of bacteria that call your gut home may help regulate everything from your ability to digest food to how your immune system functions. But scientists know very little of how that system, known as the microbiome, changes over time—or even what a “normal” one looks like. Now, researchers studying the gut bacteria of thousands of people around the globe have come to one conclusion: The microbiome is a surprisingly accurate biological clock, able to predict the age of most people within years.