There are massive developments in children's early number skills in the ages 4- to 6-year old during which they attend preschool education and before they transition to formal school. We investigated to which extent these developments can be explained by children' schooling experiences during preschool or by chronological age related maturational changes. In a secondary data-analysis of an existing longitudinal dataset, we compared children who were similar in age but different in the amount of preschool education (Old Year 2 , n = 104, M age = 62 months SD age 0.9 months vs. Young Year 3 , n = 71, M age = 65 months, SD age = 1.5 months) as well as children who were similar in the amount of preschool experience but differed in age (Young Year 3 , n = 71, M age = 65 months, SD age = 1.5 months vs. Old Year 3 , n = 104, M age = 74 months, SD age = 1.1 months). All children completed measures of numbering (verbal counting, dot enumeration, object counting), relations (number order, numeral identification, symbolic comparison, nonsymbolic comparison) and arithmetic operations (nonverbal calculation). We observed effects of preschool on object counting, numeral recognition and number order. There were also effects of chronological age on verbal counting, number order, numeral recognition and nonverbal calculation. The current data highlight which early number skills may be particularly malleable through schooling. They provide a more careful characterization of the potential factors that contribute to children's early numerical competencies. • Children show massive developments in their number skills before formal school. • We examined whether these can be explained by schooling experiences and/or age. • Effects of preschool were found on object counting, number order and numeral recognition. • Age effects occurred on verbal counting, number order, numeral recognition and arithmetic. • Preschool experiences and age affect early number skills before formal schooling. [ABSTRACT FROM AUTHOR]