At what point does tetracycline affect the dentition?

Tetracyclines bind to calcium and become incorporated into developing hard tissue (chelating with hydroxyapatite) during tooth mineralization — affecting any tooth that is actively forming/calcifying at the time of exposure, which is why tetracyclines are avoided in pregnant women and children under about 8 years old (while teeth, including permanent teeth, are still developing). It is not the moment of birth or of eruption itself that matters, but the active calcification/development window.