I just removed wallpaper that had to be about 30 years old from a plaster wall. After applying a product called DIF the wallpaper, it was easier to remove. It can be found in any big box store and you hust mix with warm water and spray on the wall. Use a scraper to remove the paper and reapply the DIF to any adhesive that remains on the wall. The glue will come off easily. Then lightly sand the wall and apply some primer before painting/repapering. Good Luck.