The older an animal is, the better the immunity, because there are less maternal antibodies interfering and a more mature immune system to respond. (aspcapro.org)
However, it is generally not a good idea to have a puppy wear it since puppies don't have a fully mature immune system yet. (mypetneedsthat.com)
Puppies and kittens do not have mature immune systems yet; plus, because they are small, fluid loss from diarrhea hits them much harder. (vin.com)