As soon as they came in.
When my daughter's were babies and we didn't have the infant toothpaste I would just give them a very soft bristled toothbrush to gnaw on after I brushed their little teeth with plain water. I didn't give them toothpaste until they were between 2 & 3 when they learned to spit it out instead of swallowing it.
Take care of those baby teeth before they come in, too, by using one of those baby gum cleaners they have now that fit on the tip of your finger....or just wet a washcloth, wipe the gums quickly, and allow the child to gnaw on it.
My granddaughter (9 months old) allows me to brush her 2 teeth with the infant toothpaste and toothbrush and then I allow her to "brush" them herself.
Developing good habits now is the key to lifelong dental care. My daughters are now 20, 18, & 15 and have never once had a cavity. :)
at 6:13 AM on Nov. 11, 2009