Man, I'm 37, have lived in the US for 37 years plus 9 months if you believe life starts at conception, and have a better command of the English language than many if not most, and I constantly have to look things up! I don't know if it means I'm actually not in any better command of the language than most 5th graders, or the things I read are written at a high level, or I just read enough to stumble upon new words, idioms, and phrases constantly, but whichever it is, I feel you :)

Of course when English has over 1,000,000 recognized words, who can be expected to know all of them?