I think it is hard to tell exactly if they are succumbing to old age. At around 2 or so, you start to see them slow down. I usually notice my older rats are not as active, they enjoy cuddling more than playing, etc.

If she seems generally healthy otherwise(besides the loss of appetite), than you could try offering her other foods. When my rats start to get old and their appetite wanes I offer higher calorie foods(baby foods, nutri-cal, etc). Basically this is the time to offer her whatever she wants, since it is not going to matter too much now. I basically let them eat whatever they wanted(within some moderation of course). My older rats loved Gerber hotdogs.

If you see weight dropping off, then you may want to go to a vet, as this is more of a symptom of something(and you don’t know what it is without a vet). Otherwise, just offer more food, and some of the forbidden fattening food.

She is probably just getting older. This is when you just enjoy the time you have left. Ease up on what she is allowed to eat, and keep an eye on her overall health.