"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?
日本語と Google Translate
6 days ago