You want your dog to be comfortable with stairs, especially if she'll need to access them to go outdoors. The exact age at which your pup will be able to navigate stairs will depend upon her size, and the height and number of the steps she'll need to climb.
Your puppy may able to handle a short step or two from the day he joins your family at about 8 to 10 weeks of age. Over time, introduce him to the rest of the stairs in your home so he is comfortable with them by 16 weeks. Even if you don't have stairs at home, it's a good idea to introduce him to stair navigation by the time he is 4 months as part of his socialization process.
To climb down stairs, you pup should to be able to comfortably reach a lower step with his front paws while his hind paws are on the step above. The safest place to begin the learning process, according to Guide Dogs for the Blind, is at the bottom of the stairs, with front paws on the ground and hind paws on the first step. Work slowly and calmly with your dog during this process, praising him often and moving up the stairs only when he's ready.