relatively invariant both for different children and for different lan- guages.But almost all children, with their limited memories, restricted reasoning powers, and as yet almost nonexistent analytical abilities, acquire perfect fluency in any language to which they are adequately exposed, and in which they interact with others.Severely retarded children, who have a slower rate of development (but in the same relative sequence), are likely never to develop a complete adult grammar for this reason.The effects of age on both L1 and L2 acquisition are dis- cussed in Chapter 4 as the Critical Period Hypothesis.