Researchers have always been concerned with innovative approaches to facilitate the learning process for language learners. One of the facilitating strategies is guided discovery learning, which can help learners to develop their critical and analytical thinking skills. Hence, the present study is an attempt to discover the effects of guided discovery learning on the development of teenage and adult learners’ syntactic structures. The participants of this study were 34 male EFL Iranian learners in Iranmehr Language Institute, Aleshtar, Lorestan province, Iran. The participants were divided into two experimental groups based on their age (12 to 18 as teenagers, and 18 to 30 as adults). First, the Longman proficiency test was given to the participants to measure their language ability. After that, a pre-test was given to the students regarding syntactic structures before the treatment. Then, every two weeks a post-test was given to the students to assess their gradual development. One immediate and two delayed post-tests were given to the students. In this study, paired and independent samples t-tests were used to compare the development of syntactic structures in the studied groups. The results of this study showed that both groups (teenagers and adults) made an improvement but adults improved more significantly than teenagers. The findings of the study contributed to the practical employment of guided discovery learning as an effective teaching methodology, especially for adults in the process of language learning
