My dog cannot stand up for some reason. He was fine a few days ago and now he cannot make his legs work for some reason. He is quite old... is this just a sign of his age?

He may be suffering from Liver failure. I would take him immediately to the Vet have him checked out to be sure. Even though he is old, you should still have him vet checked. He may be suffering, dogs don't always let you know when they are in pain. I sure hope your dog will be helped. I had a Brittany for almost 15 years and she went into convulsions and didn't come out. It was an awful death for her. We had to take her in to the vet, because they wouldn't come to our home. Believe you me, the next pup I get won't go to that vet. It was a terrible time for my husband and me. She was my husbands best bird dog. It has taken me three years to get over her. I am now ready to get another pup. Next Spring I am getting a Silky Terrier, can't wait.
Good luck with your sick buddy.