By the age of one year and eight months children know about 50 words. by age five they are learning about 15-20 words PER DAY! By age eight they know about 18,000 words. For most languages like English and the European languages the basic acquisition of a language is complete by the age of six or seven, however some languages may take longer. The Cree Native American language in Canada is unusually complex and may not be completely acquired until around age ten.

(The Great Courses – Understanding Linguistics: The Science of Language)