Children under 6 years old require help from an adult when brushing their teeth. The MOI technique, using systematic movements suited to a child's abilities, should be introduced early. As children learn through imitation, they can watch adults brush and play with a children's toothbrush from a young age. Two to three year olds can brush the masticatory surfaces with horizontal movements and later learn circular and up-down motions. The appropriate brushing technique depends on a child's age, with scrubbing recommended for ages 9 and under and Bass for ages 10 to 15. Toothbrushes should be replaced every 8-10 weeks or sooner if bristles are bent.