My boy Toby the Tyrant (Prof. Sumo Yorkie
) was only 4 months when we brought him home and he started marking immediately. I have a female who is exactly 4 months older than him, had never gone into heat and was spayed 4 days after Toby came into the house, so Im pretty sure that wasn't the cause of his marking. He seemed to do it right from the get go. We are working on it, but all Ive ever owned has been male dogs (big huge dogs) and have never once had an issue with them marking in the house until now. So yeah... I'm going to get him fixed and hope for the best.