I started 'brushing' Zoey's teeth early - 12 weeks old. I did this because I had heard they can have alot of dental issues and my breeder said she had a dog choke on a greenie. I must admit that the brushing is more her chewing on it but it does the same as brushing. Especially with the three-sided toothbrush. Sometimes I help her with pulling gently but for the most part, she does it on her own.
Maybe the product has something to do with it? I use this Cain & Able Kissable (Vanilla flavor) one available at Petco:
KissAble Doggie Dental Kit at PETCO
I usually brush my teeth (electric brush) at the same time and she loves that. I have been considering switching to the electric one available at Petsmart (see below) however, I certainly do not want to jinx her already great habit.
Petosan Silent Power Toothbrush for Dogs - Health Care - Dog - PetSmart
Her brushing has also helped with her current teething. When she gets a loose tooth, we go to brushing a bit and it helps to remove it so she is more comfortable.
Hope this helps!