Grooming older dogs is all about taking time, being careful and treating the more mature dog to the respect they deserve and demand.  A GSD at 13 is actually 96.  Less movement is important as you would not expect a human of this age to be able to do the splits - same with our canine companions. There can be ups and downs with older dogs some get very grumpy and snappy in older age but others just relax and take it in their stride.  No dog is too old for grooming, looking after older dogs is very important for their health and wellbeing.