By the time a rat is two years old, it's old. Rats rarely live beyond two years, although some have been knows to live up to four years. Ten days are equivalent to a year for a rat, so when your rat is two years old, it's 73 in "rat years." Part of being a responsible pet owner is knowing how to take good care of your aging pet.
Be extra careful about keeping the cage clean. An aging rat's immune system may not be as healthy as it was when it was younger.
Supervise your rat's outside-of-the-cage play time even more than you have in the past. Older rats are more likely to run into objects, slip or fall, just like older people. Be extra gentle when handling your senior rat.
Watch for signs of upper respiratory disease, including goopy or runny eyes or nose, and difficulty breathing. Old rats are prone to pneumonia and other upper respiratory infections, which require immediate veterinary attention.
Observe the quality of your rat's fur and skin. If the coat gets thin, or if sores develop anywhere on the body, you'll need a trip to the vet for these problems, too, which can be very uncomfortable for your rat.
Keep track of your rat's body weight. If your rat seems to be getting overweight, cut back on his food a bit. If he seems to be getting skinny, increase his food supply, but contact your veterinarian immediately. Even though some mammals (including humans) tend to lose body condition a bit as they age, weight loss may still indicate an underlying illness.
When you hold your rat, gently run your hand over his body, checking for any lumps. If you find one, don't panic. Tumors are extremely common in rats, and not all tumors are lethal. Fatty tumors, for example, are common in animals and rarely effect their overall health unless the lumps grow very large, in which case they can be removed. If your veterinarian determines that a tumor is malignant, he will help you weigh the pros and cons of treating your rat.
Watch for signs of paralysis in your pet's rear legs. In some rats, the nerves running to the legs begin to deteriorate because of age. After having your rat examined, it's possible for him to live a full life, as long as he continues to eat, drink and sleep appropriately, though he may lose use of his rear legs. You may want to change the bedding if you are using wood chips and try something softer, such as shredded fabric or paper towels. If your rat seems to be in pain or stops eating or drinking, he should see a veterinarian immediately for treatment.