This study explores developmental changes in creative thinking and lexical richness (measured through lexical density and diversity) during late childhood and examines how different classroom methodologies may be associated with these changes. Data were collected from 57 students at three time points (between 5th and 6th grades of primary school), allowing us to track the longitudinal development of both variables. Participants were divided into two groups based on their learning methodologies: Cooperative Learning (CL) and Individual Learning (IL). In the CL group, students engaged in structured cooperative activities centered on problem-solving and open-ended tasks in Language, Mathematics, and Arts, fostering peer interaction and idea exchange. A repeated measures ANOVA revealed significant age-related improvements in creative thinking and lexical richness, with the CL group showing greater gains, particularly at the second (T2) and third (T3) assessments. Correlation analyses showed a strong positive relationship between lexical richness and creative thinking, suggesting that a richer vocabulary may facilitate the generation of more complex and innovative ideas. These findings highlight the value of cooperative learning in promoting both creative and linguistic development during late childhood and underscore the need for further research into how classroom practices influence cognitive growth in primary education.