Sorry to hear about the newest loss. Domesticated rats are some of the most adorable common pets I hear about, and I place them in the category with hedgehogs that they'll really endear themselves to you while lasting maybe four years. A friend remarked about having a 7 year old hedgehog and I can only imagine the geriatric issues, 7 is a long time. But I know the pets in your menagerie get the best of care, so however many years these rats get must be the best years any rat can expect.