After consolidation is complete, at around age nine or ten, the brain loses much of its plasticity, and changes in wiring become much harder to make. People who learn to speak a foreign language after age ten, for example, rarely will be mistaken for a native speaker of that language. During the first year of life, children make all sounds of every language and, in so doing, develop neural connections that allow these sounds to be perfectly made. But if the sounds are not reinforced by adults and used regularly by the child, the early connections will disappear during the period of consolidation. Even extensive practice during later life never will recreate these original connections

The critical period for developing other skills is even shorter. Infants, for instance, occasionally are born with cataracts. It appears as if their eyes, visual nerves, and visual area of the cortex would function perfectly if only the cloudy coverings over the eyes were removed. If the cataracts are removed during the child's first two years, s/he quickly gains visual abilities and soon is indistinguishable from any child born without cataracts. If the operation occurs after a child's second birthday, however, it is useless -- s/he will never regain the ability to see. The critical period for developing vision has passed, and the opportunity for the child to see has been lost forever.

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