Great question! There are lots of psychologists who investigate this exact question. Children should have about 6 words that they can say and understand when they are 12 months old. By two years old they should be able to put two words together in a simple sentence. By three years old they should be quite fluent. However, all children are slightly different and some may know more or less words by these times.
What is REALLY interesting is that children who are brought up in homes were two languages are spoken (bilingual homes) reach the same stages at the time as children who are only learning one language (monolingual).
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blondey commented on :
Thanks very interesting!