Is my dog just slowing down?

[yuityfroghurt]

I have a poodle mix, Max, who is about 11 years old now. I found him roaming the streets of my hometown when he was around two years old (a couple of different vets thought he was around two at the time so I tended to believe their guesses). Within the last three weeks, he has slowed down tremendously. He doesn't want to come downstairs in the morning to go out or even to greet me when I get home from work (I work nights) and naps all.the.time. I literally woke Max up and carried him to his food bowl last night so my dachshund wouldn't eat two dinners (which he would happily do). He does still eat and drink normally, and is maintaining his usual weight of 17.2 lbs.

I was worried he was having some pain, he seems "stiff" when he gets up from sleeping. He sort of half limps with both of his back legs (it's hard to explain in writing) but once he gets going he seems to walk fine. I've seen him trip going up the steps a couple of times after he's been walking a little bit though. My vet had previously told us he had mild luxating patellas but when he examined him he couldn't find anything really wrong (Max didn't make a peep when he was manipulating his legs in different directions) so he started him on Carpaquin once a day for two weeks. I saw no improvement in his "stiffness". When we went back for his follow-up, the only suggestion the vet had was to build him a ramp from our patio to the yard since the step is a little high (he gave him a thorough exam and manipulated his back legs again). He had blood drawn at his first appointment and the only thing that was slightly elevated was his liver function. I started giving him a glucosamine/chondroitin supplement when all this began and he also takes synthroid twice a day for hypothyroidism. 

I've never had a dog live to be this old before, the Schnauzer I had growing up passed away at age 8 from complications from diabetes. I certainly didn't think I'd see such a drastic reduction in Max's activity level in such a short period of time though. Is this normal?