Breed aging question

My little boy Hugo was rescued from a Boston rescue organization when he was around two and I've had him almost 6 years making him around eight years old.  I'm starting to get a little down because he began to gray around his muzzle and eyes about a year ago and now it's fairly pronounced.  I have a few questions. 

First, what is the breed's average lifespan?  Secondly, Is it normal for him to gray so early?  He had a lot of stress in his life before I got him and I don't know if dogs are like humans and can prematurely gray because of stress.  I had a black lab that had a lot of gray on her muzzle, but not until she was around 13 or 14 and she lived to be 16.  He doesn't have any of the other tell-tale signs of age yet including joint problems or behavioral changes.  Any thoughts?  I love my baby and it just worries me....