Sealants are placed almost exclusively on back teeth, and usually soon after they erupt. Both choices follow from the shape of those chewing surfaces.
The grooves are narrower than a bristle
Molar chewing surfaces are formed by cusps meeting, and the junctions between them create fissures running across the tooth.
Many of these fissures are deep and narrow, with openings measured in fractions of a millimetre, far finer than the tip of a toothbrush filament.
Bacteria and food debris enter readily while cleaning implements do not, which makes these surfaces the most common site of decay in children.
Sealing converts the shape
A sealant is a flowable resin drawn into the fissure by capillary action and then set, filling the void and leaving a smooth surface behind.
With the shelter eliminated, a brush can clean the surface effectively and saliva can reach it, restoring the normal balance of loss and repair.
The material is not providing strength or replacing tooth structure, and it is not a filling. Its function is entirely about geometry, which is also why it is placed on sound teeth rather than on decayed ones.
Timing follows eruption, not age
A tooth is most vulnerable in the period after it appears, because its enamel is still maturing and it often sits below the level of its neighbours.
Partially erupted teeth are also harder to clean, since a flap of gum can cover part of the chewing surface for months.
Sealing shortly after full eruption addresses the highest-risk period, which is why the recommended timing tracks the tooth rather than the child's birthday.
Bonding requires a dry field
Resin bonds to enamel that has been etched and dried, and contamination by saliva during placement is the usual cause of early loss.
Isolation therefore matters more than the material chosen, and a cooperative child makes a considerable practical difference to the result.
Where isolation is impossible, alternative materials that tolerate moisture better may be used instead, accepting a shorter expected lifespan.
Sealants need checking, not forgetting
Sealants wear and can chip or partially detach, and a partially lost sealant leaves an edge that can trap film.
Routine examinations include checking their integrity, and repair is straightforward when a defect is found early.
Whether a particular child needs sealants at all depends on their decay risk and the depth of their fissures, which is a clinical assessment rather than a universal recommendation.