I have a dog that turned 14 last October and she just plain doesn't respond like she used to. She's got five obedience titles, four agility titles, earned her CDX (second level obedience title) with first places beating 20+ dogs in her class - and yet now she doesn't understand a simple "sit".
I think it's a combination of things. She doesn't hear nearly as well (although she DOES understand "food", "doggie bones" and "nummers" .. *L*). She's easily confused - literally confused, she'll wander around and look lost at times. And she's not nearly as limber as she used to be. When she lifts her paw for a "wave" she can't lift it very high.
So I accept that she's old, maybe a bit senile and certainly not physically what she was, and I just don't make her do much. She gets her treats just for coming inside when I call her and getting along with everyone else. Sometimes I think we just have to accept that as they get older, they just can't do the things they used to. It's very sad, I know. But it's important to just enjoy the time you have together and not fret over the old dog not being able to respond to commands.
I hug her a lot, scratch behind her ears and talk to her. And I cherish every day we still have together, because I know that her time is limited.
Melanie and the gang in Alaska