An independent American-Brazilian research project is attempting to measure English vocabulary sizes according to age and education, and whether or not you're a native speaker.
I tested myself, and they estimate my vocabulary size at around 33,000 words which, according to them, is upper range of average for native English speakers.
Interestingly, even they say the results may be skewed at the moment, with people who have participated in the survey so far being quite a bit more literate than the wider population average, and with an average self-reported language SAT score of 700. (With the cheeky comment in the FAQ that “Basically, we need more YouTube commenters participating. :)”)
They're still looking for more results, especially from children and teenagers, if you're so inclined to contribute.