I think babies are considered newborn until about 3 months, though it may depend on the context. Some things do change around 6 weeks, but I noticed a huge difference in my dd right around 12 weeks. Suddenly, she had lost her "newborn look".
If you're talking about clothing, sizes 0-12 months are "newborn" and sizes 12-24 months are "infant". So they like to mess with you, because no one would call a 12 month old, nor most babies who can wear a 12 month size a "newborn".