When I was in journalism school (longer ago than I care to say) we were taught to keep our prose at what was then considered a ninth-grade reading level.
Fast-forward 20 years (or so) and I am retraining as a technical writer -- trying to help "regular" people decipher obtuse, jargon-ridden manuals -- and my instructor says to me:
"Pretend your audience is in the third grade."
Yikes, ladies and gentlemen!
All I can say is "Yikes!"