This article aims to present two essential points. Firstly, syntactic complexity value is an indicator of language development of children, especially in relation to narratives due to maturation and increased cognitive development. Secondly, complexity is measurable and assessable. This article argues that syntactic complexity as numerically measured has benefits for studying the close development of children in different age groups in which differences of language pattern and innovation may not be readily discerned. In order for a more accurate comparison between different age groups, the information employed was narratives of Thai children in the CHILDES database, Thai Frog Story series. The age groups were divided into 4 tiers: 4, 6, 9 and 11 years old respectively. Each group contained ten children which were compared to ten adults. The study found that syntactic complexity of children’s narratives develops increasingly until it resembles to that of adults.