Picture books play an important role in children's language development. To achieve optimal language performance, picture books must be matched to the child's abilities. Age statements can be used as a guide when selecting a picture book. As such, they should indicate that there are real differences in the linguistic complexity of the text, given the age for which they are recommended. The aim of this thesis was to analyze and compare the lexicon of picture books with age ratings of 12+, 18+, and 24+ months. The analysis was performed on a sample of 18 Croatian picture books. The features of the lexicon were examined using the measures of lexical sophistication, Measure (D) and Moving-average type-token ratio (MATTR). The level of noun sophistication was compared with data from the Croatian Frequency Dictionary of Child Language (Kuvač Kraljević, Hržica, and Štefanec, 2022) and the Croatian Web Corpus (Ljubešić and Klubička, 2014). Measure D and measure MATTR were calculated using the computer program CLAN. The results of this study show that as the ages increase, the lexical diversity of picture books also increases, with Measure D found to be a more accurate measure than MATTR in distinguishing ages, which is confirmed by previous research. It also shows that the lexical sophistication of nouns decreases with increasing age classification, which is contrary to researchers' expectations. Picture books aimed at younger children have more sophisticated nouns than picture books aimed at older children. A possible explanation for these results could very well lie in the syntactic component of the text, which was not analyzed in this study. The analysis of picture book lexicon shows that picture books provide a rich source of new words that children rarely encounter in everyday life. Further research on Croatian picture books is needed, conducting both semantic and syntactic analysis of the text on a broader selection of picture books.