Well, Frisco's near 6 months, and I'm trying to take advantage since he follows me everywhere that he does what I do. I'm dabbing his brush with a sniff of Petrodex chicken and we brush together. I don't know anything about doggie toothpaste, but enzymes can't hurt anything and ANY simple physical scrubbing can't hurt either. At least it has made for a pleasant introductory experience.
You might be amused at his first and final (more realistic) toothbrush lesson videos at
YouTube - littlefrisco's Channel . They are short, just as sweet, and quite to the point.