Baby teeth are not simply smaller versions of adult teeth. Their internal proportions differ in ways that change how decay behaves and how quickly it becomes significant.
The protective layer is physically thinner
Enamel on a baby tooth is substantially thinner than on a permanent tooth, roughly half the thickness in many locations.
Enamel is the layer that resists acid dissolution. A thinner layer offers less material to lose before the softer dentin beneath is reached.
The mineral itself is also slightly less densely arranged in baby teeth, which affects how readily acid penetrates it.
The pulp chamber occupies more of the tooth
The nerve and blood supply sit in a chamber at the center of the tooth. In baby teeth that chamber is proportionally larger and sits closer to the outer surface.
Pulp horns, the pointed extensions under each cusp, reach especially high in baby molars. Decay traveling inward encounters them sooner.
Combined with thinner enamel, this shortens the distance between a surface cavity and the tissue that produces pain and infection.
Newly erupted permanent teeth are also vulnerable
Enamel continues taking up minerals from saliva for a period after a tooth erupts, a process sometimes described as post-eruptive maturation.
During that window the surface is more porous and less acid-resistant than it will later become.
This is part of why the first permanent molars, which arrive around the time many children are managing their own brushing, are a common site of early decay.
Why decay can progress quickly
The same rate of mineral loss covers proportionally more of a baby tooth in the same time. A lesion that would be shallow in an adult tooth may not be shallow in a child.
Decay between baby molars is also difficult to see directly, because the contact surfaces face each other. Radiographs are how those surfaces are examined.
Progression rates vary widely between children depending on diet frequency, plaque control and fluoride exposure, so timing is assessed individually.
Why baby teeth are treated rather than ignored
Baby teeth hold arch space, contribute to chewing and speech, and are present for years before shedding. Infection in one can affect the permanent tooth developing beneath.
Treatment decisions weigh how long the tooth is expected to remain against the extent of decay, which is a clinical judgment specific to each tooth.
Regular checkups exist so that these lesions are identified while they are small, and parents should raise any visible spot or complaint of discomfort with a dentist.