Start brushing your dog's teeth between the ages of eight and sixteen weeks,” says Sievert. “Plus, when brushing them this early, [your dog] becomes used to it, and it's much easier to make it part of his daily activities.” Even when you’re starting with a puppy, though, start off slow to get your dog used to having their mouth touched and their teeth brushed. “Try to avoid brushing when they are actively teething, as they may then associate brushing with pain,” says Simon. “Those who are teething may have red gums, bad breath, and a dislike of their mouth being touched.