We have had 2 minature doxie's that we adopted at ages 7 and age 14. Both were housetrained with minimal effort. We were also told not to let go up and down more than 3-4 steps as long flights of stairs weren't good for the back either. We lifted them on and off the sofa right from the beginning. One of ours did develop back problems that the vet thankfully treated successfully. Our vet would take us at a moment's notice with no appointment so that swelling and damage as a result of swelling could be decreased. There is also a correct way to lift them to keep their backs straight. Do not encourage them to stand up on their hind legs as many people do with other dogs when they teach them tricks.
Common symptom -- laying and not moving around, not getting up when you call, not even to eat. Yelping if you try to pick them up. It is very painful for them. Important to get them to the vet ASAP to minimize damage to the spinal cord.
They were the most loving little dogs.