I have a beagle/pug mix, although he's mostly beagle. I never have gone by the life expectancies of dogs from books, websites, breeders, etc..because I think it all depends on the dog. A dog could die at a few months old, or at 18 years. My previous dog was a maltese (their life span is said to be 13-16 years or something) and she only lived till 10! Although beagles average life span is said to be between 12-15 years from most books.

As for what to expect, well..I never really thought dogs go downhill that fast, but they really do at least in my experiences. Not to worry you though, just enjoy all the time you have left with her, as with all your dogs no matter what age. What happened to Sassy was that she got arthritis at around 8 and it didn't get bad until 9 1/2 when all treatments stopped working. By the end, 10 years old, she couldn't even get up anymore so we put her down to take the pain away. It was very hard to watch but it was the best thing for her. Not all dogs get arthritis though, but that was just my experience.