The Mongolian Bankhar Dog Project. This makes interesting reading as these are a large breed and examples of 15-18 year old dogs working with Nomads in the field is not uncommon . A typical meal being boiled rice, sheep innards and cheese whey. http://bankhar.org/?page_id=48
That sounds like a good balanced diet. 15 and over for a big dog is good going. Interesting reading 
A Bankhar dogs have traditionally performed.