The window of opportunity for learning language refers to a time period when it is easier to learn language skills at higher levels. The blue curve on the graph to the right shows that the number of new synapses, or brain connections, for language acquisition peaks around 11 months of age.* This does not mean that children cannot learn language skills when they are older. It means that there is a period of time when it is easier to learn at a high level.
Citation
Nelson, C. A. (2000). In From Neurons to Neighborhoods: The Science of Early Childhood Development. Shonkoff, J. P. & Phillips, D. A. (Eds.), P.188, Washington, D.C., National Academy Press.