hi. in terms of the differences between children and teenagers learning language, teenagers are around puberty period. they face a lot of physical and cognitive changes. regarding their cognitive resources, they can understand abstract concepts better. this makes them to understand abstract notions which are taught in their language classes. in terms of identity, thereis another GAP between them. teenagers have a kind of first language and culture identity formed and established . children are more flexible and learn better due to unfixed cultural and linguistic identity. best

Thank u. I need some empirical researches done in this srea.

This would be a good starting source: Muñoz, C. (Ed.). (2006). Age and the rate of foreign language learning (Vol. 19). Multilingual Matters. It presents empirical evidence that in immigrant contexts adolescents and young adults are initially faster learners than young children.