Why is c. diff most dangerous for a young or elderly patient?

Can be life threaten: C.Diff colitis causes diarrhea and dehydration. But what is important is that it can cause the colon wall to be sick enough to lose integrity and infection from the stools can go to the blood causing sepsis. In these rare severe cases the life threatening nature of the disease may require removal of the whole colon for the person to survive.

Dangerous in elderly: Older age is a well established risk factor for having a more severe case of c diff, as well as dying from it. This is likely because being older is a surrogate marker for having other diseases that can suppress your immune response, such as diabetes, cancer, etc. The very young (2yo and under) have not been shown to have more severe cases of c diff. In fact, many infants are colonized with it!