D

The mineralized surface of erupted enamel is not inert; it constantly undergoes cycles of de- and remineralization driven by salivary ions and plaque fluids. These physicochemical exchanges—essentially ionic diffusion of Ca²⁺, PO₄³⁻, F⁻ and other ions—persist for as long as the tooth is present in the oral cavity, i.e., throughout life, not just for a few years after eruption or until adulthood [Abou Neel et al., 2016, PMID 27695330; Borovskiĭ & Leont'ev, 2002, PMID 12532883].