I've been having some fun today playing around over at Test Your Vocab, a site that tests your vocabulary (the words for which you know at least one definition), and then runs some numbers to tell you whether you're above or below average for your given age and intelligence level (as measured by self-reported SAT Verbal score).
According to the test, my vocabulary is above average for my age, but actually below average when you add my SAT score into the equation--maybe I just got lucky back in high school, or maybe I did well on the SAT Verbal because of my command of grammar and logic, rather than gross vocabulary size.
Here's the two main charts that I mentioned, in case you're curious. I think it's interesting to see how quickly our vocabulary grows, and then how significantly it levels off, regardless of your overall intelligence.
The one main caveat that the site's organizers present is that the average test-taker's general intelligence level is much higher than the overall national average--self-reported SAT Verbal scores average about 700 for the test-takers, as opposed to 500 for all SAT takers nationwide (and an estimated 350 average if all Americans were to take the test).
So take these results with a grain of salt, but feel free to test your own vocab against their charts. Maybe you'll even surprise yourself.