Hi Jo C-
AAFCO does not have a nutrient profile at this time for a senior dog. Senior dog formulas are simply an adult maintenance diet, usually with lower protein, fat and calories. Many here, including myself do not use a senior formula for their senior pets. For my 8 year old Lab, I simply monitor his calories closely and make sure that he maintains an ideal weight.
Keeping senior dogs (and really any dog of any age) at at 4/9 on the Purina Body Condition Score, is of uttmost importance. This keeps stress off their joints, bones, heart etc. According to research done by Purina, a dog kept at ideal weight can have their life extended by 15% (1.8 years). I would say this is one of the most important things to help keep senior dogs healthy.
What are you feeding currently? Is she healthy? Good weight or overweight? Good bloodwork? 13 is a very good age for a pitbull. It sounds to me like whatever you have been doing thus far is working well.
