My dogs have wagged their tails from the time we've gotten them as puppies. I've seen videos of very young puppies (still with their mother) wagging their tails while playing with each other or with people.
Keefer 8/25/05-4/24/19 ~ The sweetest boy
Halo 11/9/08-6/17/18 ~ You left pawprints on our hearts
Dena 9/12/04-10/4/08 ~ Forever would have been too short