I asked awhile ago if vocabularies continue increasing with age. A reader sends a link to a website, testyourvocab.com, that offers a 120 word vocabulary quiz, ranging from extremely easy to extremely hard. It's not a multiple choice quiz, you just have to be honest with yourself about whether you know at least one definition for each word. Then it offers an estimate of the size of your vocabulary out of what the authors consider to be the 45,000 words that comprise the full, non-technical English vocabulary. I got an estimate of 40,100, which sounds reasonable: I know a lot of words, but I lack the precision of mind to rack (wrack?) up the really big numbers.
From looking at the ages of the self-selected sample of vocabulary test lovers who took the test (average verbal SAT score 700), they found that people from 15 to 29 add about a word a day to their vocabularies, with slower increases perhaps into your 60s. The age slopes are about the same for each SAT score. They need to adjust for the 1995 recentering of verbal SAT scores, but, still, they've got pretty good evidence that vocabulary size improving with age is a real effect, not caused by self-selection in their sample.
They also print English vocabulary sizes for non-native speakers of English by country. At the top of the list, not surprisingly, are Danes, Norway, Finland, Sweden, and Belgium. (My one trip to Belgium in 1994, I noticed that a fair number of people spoke English beautifully, with nice near-English accents — I mean, aesthetically speaking, they spoke better English than I did). Germany is well down the list. I presume it's a big country and people don't feel as much need to learn a foreign language as in small countries where they need English to speak to people from other countries. At the bottom of list is Iran. All this is self-selected, but sounds plausible.