kiasikiasu wrote:As above.
your kid or your friend's kids?
typically, it depends.
as soon as the kid has teeth, it is important to start cleaning it.
if you bear to part some $10+ for a small tube of toothpaste that can be swallowed, typically your kid can start brushing anytime.
if not, wait til you had trained your kid how to spit out toothpaste, cos constant ingestion of toothpaste with flouride content is no good for kids.