There is a lot of research going into the human microbiome, how it tends to change as we age, and the factors that affect those changes.
For example, our microbiome is directly affected by the way we are born, the microbiome of those with whom we spend the most time, what we eat throughout our lives, physical activity, toxins, and much more.
Check out this article for an in-depth review: https://todayspractitioner.com/microbiome/the-ever-evolving-microbiome-ages-and-stages/