Children start to develop number sense even well before they start the school. Developing number sense serves as an intermediate tool for learning conventional mathematics taught in schools. This number sense has three key areas: number knowledge, counting and arithmetic operations. As a result, the aim of this study was to examine aged related complexity of number sense development of young children's aged four, six and seven under two key areas: number knowledge and counting. Semi structured task based clinical interviews were employed to examine number sense development. Five different assessment tasks were employed with three children. Children's responses were analysed to identify their level of number sense understanding and difficulties with developing number sense. Findings were reported under two categories: first children's ability to understand number concept and their ability to accomplish number word sequences and second counting. Findings of the study indicated a significant age related complexity and improvement in both two aspects of number sense. Older children with more experience developed better number sense than the younger children.