Dog Arthritis Treatment

I adopted a 13 year old dog from the pound (they were about to put her to sleep) and she has mild arthritis. I give her soft pillows to lie on and let her be on the couch but she still seems to move stiffly and lies around a lot. Is there any special food I can give her or anything? I have never had an old dog before.

what is the average age for a dog to begin to start showing signs of arthritis, and what are some signs?