actually, shih tzu are long lived dogs. small dogs live up to 5 years longer than some big dogs.
Yeah, I agree that dogs get inactive as they getbolder, like people do. maybe he has doggiwe rheumatism. Look for disorientation, and being startled at slight noises. Shih tsu are prone to eye and nose problems because of their squishd faces. I would moniter his behavior, and give him lots of love. Dont put j=him down unless he is in serouis terminal pain

dogs are my forte