This study covers phase IV of a prospective clinical long-term study. Objective of this clinical investigation was to analyze the effects of a long-term prevention program on dental and oral health of teenagers at the age of 13 to 14 years. The entire study was subdivided into four phases. Phase I comprised an individual preventive care during pregnancy (“primary-primary prevention”); phase II assessed mothers and their young children until the age of 3 years (“primary prevention”); and in phase III, mothers and children at the age of 6 years were investigated. In phase IV of the study, the oral health of 13- to 14-year-old teenagers was examined (13.4 ± 0.5 years; n = 29). All phases consisted of an examination, education about oral health care, and treatment based on the concept of an early oral health care promotion. The control group consisted of randomly selected adolescents at the same age (n = 30). The following clinical parameters were assessed: decayed/missing/filled teeth (DMF-T)/decayed, missing, and filled surface teeth index, hygiene index, papilla bleeding index, Periodontal Screening Index, and Streptococcus mutans/Lactobacillus concentration in saliva. The teenagers of the “prevention” group of phase IV of our prospective study revealed a share of 89.7% caries-free dentitions (65.5% sound; 24.2% caries-free with fillings). Mean DMF-T was 0.55 ± 1.0. The control group showed a significantly higher mean DMF-T of 1.5 ± 1.5 (p < 0.05) and revealed 56.7% of caries-free dentitions (30% sound, 26.7% caries-free with restorations). Our data clearly document that an early oral health care promotion starting during pregnancy may cause a sustained and long-term improvement of the oral health of children.