There are many lifestyle and biological factors that can influence the gut microbiota, demonstrating the need for studies investigating the role of probiotics in a variety of population groups.
Age is a biological factor that is associated with changes in the gut microbiota. In particular, older people often have reduced microbial diversity. The reason why this occurs is an area of research in itself, but a wealth of probiotic studies have been conducted in older subjects.
Athletes are another group that are often the subject of probiotic research. Their high-intensity training and competition load, and frequent travel, can result in immunological changes that may increase their risk of illness. In fact, the International Olympic Committee recommends probiotics for athletes to maintain immunity.
Here you can find research describing the effects of L.casei Shirota in these population groups.