Norwegian Lundehunds stop growing around nine months old. These small pups typically reach their full height by about six months, and then they will continue to fill out and build muscle mass until they reach nine months of age. Once your Norwegian Lundehund pal completes most of their rapid growth around the six-month mark, you can begin transitioning them to adult food. It’s best to make this switch over the course of a week to avoid any gastrointestinal upset.