If you look at the several mechanical measures of reading level they all boil down to counting the number of syllables in each word and number of words in each sentence. More in each is labeled a "higher" reading level. I'd instead label them LOWER, because it's often the shortest words and sentences which have the most emotional impact and the most vivid images.
Four score and seven years ago ...
It was the best of times and the worst of times,...
He was an old man who fished alone in a skiff ...
I don't know about the last one for sure, but the first two are the beginnings of long sentences. "Four score and seven years ago, our fathers brought forth on this continent a new nation, conceived in liberty and dedicated to the proposition that all men are created equal." (And the rest of the Gettysburg Address is composed of several very long sentences, too.) "It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, etc." (Granted, this is comma splicing at its finest, but still.)
Sorry, I'm weirdly pedantic today.
In any case, I write at a *decidedly* adult level. I use any big word that fits and do not hold back, and nor do I simplify sentence structure. My intended audience can handle that, and I think that people at a lower reading level can catch on and figure out any unfamiliar words from context and/or a dictionary. If not, then . . . you may not be the intended audience.