I am not sure if 13 years is old for a cocker spaniel. I think getting to 13 for some dogs is great and I hope you continue to have some more great years with her but I am sure she is just getting tired and is a "senior". My moms boxer is about 7 years old and she is getting greys and really her whole personality has changed, shes gained some weight and doesn't move around as well as she used too but she still gets her little playful moods. I've been going over to play with her more lately and yesterday when I stopped by she was so happy to see me she did the exact same dance Stormy used to do for me! She was wigging her butt so hard she knocked the table over! lol I hate to think that Maya ( the boxer ) will not be along for much longer we rescued her when she was 9 months old she was at the shelter since she was a pup and we drove 9 hours to get her as well as the shelter drove a good 8 hours to meet us. Come to think of it maybe it even was a puppy mill? Who knows I found her on Petfiner.com and they had alot of different dogs of all different breeds and ages. She has been really healthy besides some allgeries to food.
I also feel so blessed to have been able to rescue my baby and I sure hope she enjoys laying around the clouds waiting for me to come rescue her once again!