My golden retriever/brittany spaniel was three when I adopted him two years ago. He was getting sick in the shelter environment. He healed very quickly at home, and he's a perfect house pet. He's a bit headstrong about walking behavior, but that challenge is good....for me and him.
Summary....if a dog is right for you, the dog will let you know. If you're a dog person, having a perfect dog companion will keep you young!
Our first rescue was a 4-year old golden/Brittany mix. She had a great personality and was such a joyful spirit. We still laugh when we remember her antics (and weep a little, too)