Some people start as soon as the first tooth appears but in all honesty I didn't start until mackenzie was about 1. I just used water with a brush at first until she was old enough to understand to drink to get rid of the toothpaste (even now at 2.5 its a struggle to get her to spit the water out ie rinse - it just turns into a big joke when shes really drinking). I also don't make a big deal out of it when she just sucks the toothpaste off her brush as she goes thru phases where she will brush really well and then a few days later it will be back to just a quick scrub of the fangs! I also started getting her use to a toothbrush when she was in the bath - just to get use to putting it in her mouth etc when she was about 9 months old.