The perception and manipulation of spatial information are essential for mathematical learning, and research highlights gender differences in spatial abilities. The present study contributes to the question of whether these differences are evident at earlier ages and how they interact with task complexity in mental rotation. We developed the 'MEntal Reflection and ROtation' (MERRO) test for group testing in elementary schools, distinguishing between difficulty- and complexity-generating factors, such as context, dimensionality, rotation angle, and number of rotation axes. A pre-study with 148 secondary school students validated the MERRO as suitable for assessing mental rotation skills with another available instrument appropriate for that age group. The main study, involving 387 primary school students (grade 1-grade 4), identified difficulty-generating and complexity-generating factors and investigated potential early gender differences in spatial abilities. Results revealed that stimulus characteristics significantly impact task complexity, and that these characteristics partly interact with gender. These findings offer new insights into the nuanced development of spatial abilities in early mathematics education.