I think there is more to 'knowing' a word than simply recognising a group of letters which can be played in a word-game or entered as an answer in a crossword puzzle.
I think 'knowing' a word should imply knowing, or at least having a fair idea of, what it means.
A couple of weeks ago, shortly after querying the common status of prithee, I asked two friends, educated native speakers, one Brit and one West Virginian, if they knew the word prithee. They both knew it to be 'old-fashioned and Shakespearean', but when I asked them what its nearest equivalent was in modern English, one thought it mean 'sorry' and the other thought it was a greeting of some kind.
In fact, its closest modern-day equivalent is 'please', from 'I pray thee'.
Did my friends 'know' the word or not?
Top-level Scrabble players 'know' hundreds, possibly thousands of words, especially short ones, which are not in my vocabulary but which they are able to use in championship competitions.
Kids in spelling bee contests are asked to spell words such as 'psittacosis' and 'homunculus', which they can do amazingly well, having learned them from printed sheets.
They 'know' the words, they can spell them, but could they use them in a meaningful sentence?
As for TRex's comment about young people not understanding the wonderful English of the King James Bible, I'm not surprised.
Few people these days, not just youngsters, seem to be able to read and comprehend anything which is more than 140 characters in length.