A small animal, in like manner to a mouse has a two year life span and human has a average of 78 years in the U.S. Therefore, testing on animals in the last year or two of their life would leave them healthy and young. Now, some people can say that it is cruel to test on a animal with such a small lifespan, meanwhile we can go through many generations in a matter of time, thus we could see the progress faster. Most rodents reproduce rather fast, about 5-10 litters a year with 3-14 young at a time. So it would be easy to repopulate if they were endangered. Humans can not produce that fast.
Another reason that animal testing is better than human testing, is that animals have a similar body in internal and external body features. In fact, many veterinary medicines are the same used in human medication similar to antibiotics, pain killers, and tranquilizers. Having the animal bodies similar to ours, we have penicillin, insulin, and anesthetics, thanks to animal testing. We have figured how to do kidney, heart, hip transplant. Scanning techniques such as the CT and MRI where developed using