Lexical development is a key facet of later language development. To characterize the linguistic knowledge of school age children, performance in the written modality must also be considered. This study tracks the growth of written text-embedded lexicon in Catalan-speaking children and adolescents. Participants (N = 2161), aged from 5 to 16 years produced six different texts: a film explanation, a film recommendation, a joke telling and definitions of a noun, a verb and an adjective. The resultant corpus of 11,332 texts was analyzed using four distributional measures of lexical development: word length, lexical density, use of adjectives and nominalizations. Heylighen's F-measure of level of text formality was also computed. Word length, use of adjectives and nominalizations were powerful indicators of lexical development. Text type and home language had an effect on these measures. Lexical density showed no clear developmental change, and did not vary by type of text. Heylighen's F-measure was a weaker developmental indicator. Educational implications are discussed.