What is the age limit on a Liver Shunt Surgery?

I have a 5yr old Corgi with a liver shunt. I found out about it when he was a puppy and had blood work done before he was supposed to get neutered. I brought him to a consultation at Angell Memorial Vet Hospital in Boston, MA. to discuss the possible surgery. i oppted not to do the surgery and treat him medically because the risk scared me to much and the fact that you can't really sedate a liver shunt dog I didn't want to take the risk, if anything where to have happened to him I would never have forgiven myself. He is now 5yrs old and within 4months has had to be upped on his Lactulose supplement, which the vet said that the lactulose wouldn't work forever. I've been trying to do alot of research and every site I've looked at has said Surgery is the best way to go. I plan on having another consult with another Vet where I work, but does anyone have any idea how old is to old for this type of surgery and are there more risks because he's now 5? I currently have him on the Lactulose 3x per day,and Denosyl once a day, and he get's the Science Diet L/D (Liver Diet). Anything else I could give him that would help his liver and get rid of the toxins in his system? Also the vet never really gave me an estimate on the expected life span without surgery, Does anyone have any range or idea on the expected life span of a liver shunt dog without surgery and being treated medically?