How could be marking this early. Some dogs start earlier than others, same as with leg lifting. Max was lifting his leg at 13 weeks.
Having more than one dog in the house probably brings out the urge to mark earlier.
Teeth: Max was neutered at 6 months of age and had about 6 baby teeth that had to be pulled. If I recall correctly, his adult teeth were already coming in at that time.
My neighbor had a small girl Yorkie who had to have all of her baby teeth removed manually at 1 year of age. She didn't lose a single one naturally.