This study presents an attempt to contribute to the field of Education for Sustainable Development (ESD) by conceptualizing systems thinking skills of four- to six-year-old preschool children with the role of age in this particular skill. For this purpose, we developed and tested a method and instruments to assess and conceptualize systems thinking skills of 52 preschool children in early childhood education contexts from Turkey and Germany. By employing qualitative case study research, we concluded that the young children showed some signs of complex understanding regarding systems thinking in terms of detecting obvious gradual changes and two-step domino and/or multiple one-way causalities, as well as describing behavior of a balancing loop. However, their capacity was found to be limited when it comes to detecting a reinforcing loop, understanding system mechanisms by acknowledging the unintended consequences, detecting hidden components and processes, demonstrating multi-dimensional perspective, solving problems through high-leverage interventions, and predicting the future behavior of the system. Age had a notable effect on the total systems thinking mean scores of the participants.