"Native speakers' vocabularies vary widely within a language, and are especially dependent on the level of the speaker's education. A 1995 study estimated the vocabulary size of college-educated speakers at about 17,000 word families, and that of first-year college students (high-school educated) at about 12,000."
Source: wikipedia http://en.wikipedia.org/wiki/Vocabulary
Source: Japanese wikipedia http://ja.wikipedia.org/wiki/%E8%AA%9E%E5%BD%99
"When completing 6 years of age, children have a vocabulary size of around 5.000 to 6.000 words. At 13 years old it is 30.000+. At 20 years old, it is around 45.000 to 50.000 words, according to the result of the research."
I'd say these numbers are pretty consistent with what I read before.
For some reason, Japanese vocabulary is larger than English's. Maybe because it is easier to learn new words in Japanese, after you know the language well enough?
1 day ago