Why is your dog unable to climb stairs anymore?
There could be many reasons why a dog is unable to climb steps anymore. The dog may have arthritis or a bad hip. A vet will be able to determine the reasons why the dog can't go up stairs anymore.
Possibly the dog has had a bad experience with stairs in the past. Often all it takes is one traumatic event to create a permanent impression on a dogs mind. It should be possible to work through this by helping the dog climb stairs with a lot of encouragement from you. Help the dog feel safe and assure it that it is alright and that stairs are a good thing. Treats can help along with…
Your dog may be chewing on the stairs because he is teething. If your dog is a puppy this is a likely cause. When you see the dog doing this, say no and give the dog something safe to chew on. Your dog may also be chewing on the stairs because he is bored. Try playing with the dog, and giving him interesting toys to play with.
If by 'average' you mean average in age, then the answer is 'yes.' Most dogs can learn to climb stairs without too much difficulty. However, for long-torso breeds like Dachshunds, it is really advisable not to encourage stair climbing. These breeds are much more vulnerable to spinal injuries. As your dog ages, you also need to keep an eye out for signs of arthritis and slowing down. Older dogs often need mobility assistance.
Many (most?) dogs do walk up and down stairs. Some dogs avoid stairs because of a bad incident associated with stairs -- being punished or yelled at for getting on then stairs, falling on the steps when a puppy, etc. But, it is not a natural fear and with patient training a dog that has developed a fear of stairs can overcome it.