The purpose of this study is to investigate the developmental writing patterns of English as a foreign language (EFL) students across different score levels through a TOEFL Junior® writing task. The sample of this study comprised 2,016 young adolescent EFL students, ranging from 10 to 15 years old, who took the TOEFL Junior test. We performed a detailed analysis of student essays to identify possible patterns in the development of young adolescent EFL students' academic writing ability. Linguistic characteristics, such as lexical, syntactic, and discourse features, as well as organizational and content features specific to the argumentative writing genre, were examined by using automated writing evaluation tools and human ratings. The results of descriptive statistics, significance testing, and regression analyses yielded notable patterns for linguistic accuracy and complexity variables. Certain linguistic and content-related features also were found to be highly associated with the writing quality of the young EFL students in the sample. Implications for the instruction and assessment of young adolescent EFL students' academic writing ability are discussed.