My dog Tess is 13 though in good health up to last year, is now aging quickly. Your Shepherd has lived a long time already. The larger the dog the shorter the life span.
Larger breeds tend to age quicker than smaller breeds.
Great Danes, for instance, are fairly old at age 8 or 9 years, and few live beyond 11 - 12 years of age. Also when they start going downhill it happens pretty fast. If she reaches a point where her quality of life is undesirable, don't let her suffer. His last summer, just before he passed, he got to where we had to have help standing up. His hips were weak.