My inlaws had two mutts that were Lab + something else.
They just put poor Bud down a few weeks ago. One of the nicest, well behaved dogs I have ever known. He was 15. He was riddled with cancer and could barely walk by that point (bad hips). They probably should of put him down a few months sooner, but he was just such a happy/content dog, and I don't think he suffered much at all. Poor Bud.....
They also have another dog Bubba. I believe he is a lab/german shep mix. He is about 12 and the laziest dog I have ever met in my life. He does nothing more than eat, go to the bathroom and sleep. He considers it a huge inconvenience to even get up and great you when you come over. He appears to be in decent health though, no problems are apparent yet.
I would expect about a life expectancy of at least 12 years for a lab. I hope you can give the dog a good home. It is so sad to see older dogs in shelters as they have a very low rate of adoptability.