inscrutable - in-SCROOT-able - mysterious, unfathomable, enigmatic, hidden
Example: "The humor in the movie 'Funny People' will be enjoyed by younger viewers while it likely will become inscrutable to anyone over 35 years of age."
As a starter: and as a former teacher in English, I hope to add to reader's vocabulary by presenting a word a day in simple terms that anyone can understand - from current use in publications, radio,TV, etc. Remember: use a new word three times in a day and it's yours forever!