These proceedings contain the papers of the 21st International Conference on Cognition and Exploratory Learning in the Digital Age (CELDA 2024), held in Zagreb, Croatia, from 26 to 28 October 2024 and organized by the International Association for Development of the Information Society (IADIS). The CELDA conference aims to address the main issues concerned with evolving learning processes and supporting pedagogies and applications in the digital age. There have been advances in both cognitive psychology and computing that have affected the educational arena. The convergence of these two disciplines is increasing at a fast pace and affecting academia and professional practice in many ways. The CELDA 2024 Conference received 66 submissions from more than 25 countries. Each submission was reviewed in a double-blind review process by at least two independent reviewers to ensure quality and maintain high standards. Out of the papers submitted, 40 were accepted as full papers for an acceptance rate of 61%; 14 were accepted as short papers and 1 was accepted as a reflection paper. In addition to the presentation of full, short and reflection papers, the conference also includes one keynote presentation from an internationally distinguished researcher: Dr. Panagiotis Kampylis, Greek National Documentation Centre, Greece. [Individual papers are indexed in ERIC.]