Vygotsky measured his 'zone of proximal development' in years. To do this, he needed a scheme of age periods, and a set of tasks that could diagnose the next age period without defining it. In this paper, we compare the age periods in his late lectures with Halliday's categories of logico-semantic expansion as used by three adolescent writer/speakers. We find that the tendency to elaborate and embed clauses grows with expertise, while the tendency to tell stories wanes. We take this as evidence of the development of synoptic-dynamic complementarity in adolescents--and also in the theories of Halliday and Vygotsky.