Lisa Ryan & David Singleton 
Language Acquisition [PDF ebook] 
The Age Factor

This book examines the evidence relative to the idea that there is an age factor in first and second language acquisition, evidence that has sources ranging from studies of feral children to evaluations of language programmes in primary schools. It goes on to explore the various explanations that have been advanced to account for such evidence. Finally, it looks at the educational ramifications of the age question, with particular regard to formal second language teaching in the early school years and in 'third age’ contexts.