Although the size of a child's vocabulary associates with language‐processing skills, little is understood regarding how this relation emerges. This investigation asks whether and how the structure of vocabulary knowledge affects language processing in English‐learning 24‐month‐old children (N = 32; 18 F, 14 M). Parental vocabulary report was used to calculate semantic density in several early‐acquired semantic categories. Performance on two language‐processing tasks (lexical recognition and sentence processing) was compared as a function of semantic density. In both tasks, real‐time comprehension was facilitated for higher density items, whereas lower density items experienced more interference. The findings indicate that language‐processing skills develop heterogeneously and are influenced by the semantic network surrounding a known word.