<p>This study investigated whether adherence to the World Health Organization (WHO) recommendation of six months exclusive breastfeeding (EBF) is associated with better cognitive development throughout early childhood in Germany. Furthermore, it was assessed whether this association differs between low-risk infants and “small vulnerable newborns” (SVN; infants born preterm and/or with low birth weight), and whether an optimal EBF duration for cognitive outcomes exists. Data were drawn from the German National Educational Panel Study (NEPS), including up to 2511 children assessed repeatedly from infancy to 7 years. Standardized cognitive outcomes included abstract thinking (3 years), and phonological working memory (3, 6, and 7 years). EBF duration was categorized into no EBF, short EBF (i.e., EBF for less than 6 months), and 6-month EBF. No cognitive benefits were found at any age for 6-month EBF compared to shorter EBF durations or no EBF. For SVN, no consistent benefits of longer EBF were observed. Maternal education predicted cognitive outcomes consistently (range η² = 0.01–0.04). Generalized additive models indicated no optimal EBF duration across domains. In conclusion, WHO 6-month EBF was not linked to better cognitive performance in both low-risk infants and SVN in this collective.</p>