There is no set age for when a puppy begins to exhibit signs of thrusting and mounting. A dog is not of a sexual nature until it reaches the age of sexual maturity, which can be anywhere from 6 to 18 months. Puppies younger than 6 months may attempt to mount other dogs, toys or people as a form of play, due to excitement or to experiment with sexual behavior.
Dogs that reach sexual maturity do mount and thrust as a means of sexual behavior. A dog may exhibit signs of thrusting even after being neutered. Mounting behavior is common when a dog is excited, wants to show dominance, desires to play or has built up a compulsive habit of thrusting.