As a general rule, dentists suggest you brush and floss your child's teeth for him until he's coordinated enough to tie his own shoes, usually around age 6. Even at that point you should still remain involved to be sure he's brushing properly. A kid as young as 18 months may try to grab his toothbrush and want to brush by himself and it's fine to let him practice, as long as you follow up with a proper cleaning afterward. Don't forget to help with the flossing -- although it can be tough to get your child to stand still for it, it's the only way to slough off the plaque that accumulates between teeth and below the gums (areas where a toothbrush can't reach and decay often begins). If you don't want to fumble with those long strings of floss, buy a Y-shaped tool that holds the floss in place and makes the process easier. Some companies even make them in fun animal shapes.