2-3 is a very reasonable expectation, at about 2-3 years rats legs begin to weaken and they have difficulties getting around in multi-leveled cages and will need the asistance of ramps or a single level cage with everything within their reach. Myco, or respiratory ailments, are another very common illness for rats which deteriorates their health as they age, it is not a contageious ailment, all rats (with the exception of quarentines labratory rats) carry this bacteria. it depends on the specific rat wether it effects them or not, some rats will never have so much as a sneeze while one of their cagemates may weeze horribly (sounds like when you drag a straw in and out of a fast food drink cup) the next most common ailment is tumours, rats were domesticated for labratory use, therefor all pet rats come from labaratory strains, which were used for their rapidly deviding cells, they were bred to develop cancer readily. While females commonly get mammory tumors beginning around 1 1/2 years of age, these are usually non invasive and easily removed. Males are less likely to develop tumors but when they do they are much more life theatening (brain, and genital tumors are common).