In the picture below, you will see the side view of a child’s skull. I would estimate this child to be about 3 years old, give or take. Some of the jaw bone has been removed to show the adult teeth developing. As you can see, the crowns of the adult teeth have already been formed. Even the 12 year molar (the last tooth visible) has formed most of its crown and it wouldn't have come into this person’s mouth for another 9 years!
Moral of the story is that ...
It’s never too soon to start thinking about your children’s adult teeth. Get them fluoride early and continue it until the dentist advises you to stop.
Dr. Jim Ellis, DDS
1220 33rd Street Suite C
Ogden, UT 84403
(801) 783-3490
SOURCE: http://www.bestogdendentist.com/2013/01/when-do-adult-teeth-form/