As soon as their first tooth erupts, which is around 6 months. Prior to this you should use your finger or a washcloth to rub on top of their gums to get them used to the sensation. And even after they grow a bit older and are able to hold and use a toothbrush themselves, they probably aren’t doing a very good job (missing spots, or not taking enough time) and will need a bit of “touch-up” help from mom or dad.