Question: I was writing the other day and I used the word "ubiquitous". 

While I don't think "ubiquitous" is the most egregious example of "Using Big Words To Sound Intelligent", and would be perfectly acceptable in most novels, it's just, my novel's demographic would be young teenagers, and I definitely did not know what ubiquitous meant when I was a young teenager.

Personally, when I come across an unknown word in a book, I have to take a minute to research what that word means, taking me away from the book.

On the other hand though, this could be seen as a teaching moment where I can teach young readers the meaning of new words

Answer: 
The easiest way to do this is have a character use it, and another character (like yourself, not knowing the word at that age) ask what it means, or look it up, or otherwise figure out what it means.

You can even use this as a moment of conflict, or humor.

> 
> "It's ubiquitous," Angela said.
> 
> 
> Kevin frowned. "What does *that* mean?"
> 
> 
> "It means it's everywhere."
> 
> 
> "Then what's wrong with saying *everywhere*?"
> 
> 
> "It doesn't have a Q in it, does it?" Angela said, as if this was too obvious to need to be said.
> 
> 
> 

EDIT: @TheNovelFactory remarks: *This is okay for one word, but the book will start to sound a bit strange if you do it multiple times...*

Correct. You shouldn't tell the same joke twice. But the dynamic between Angela and Kevin in this example can be sustained and grown: Angela likes big words, Kevin doesn't -- at first.

Make it a rivalry, like a sibling competition. A short list: Kevin tries to stump her with a big word and fails. Then succeeds, but she is delighted. Then guesses a meaning wrong but sticks by his guess, exasperating her. Then guesses a meaning right, and she is impressed. Beats her to the punch in using a big word correctly, and she laughs.

I can even key a plot point off this rivalry: Angela uses an obscure word as a code only Kevin will get. she shouts it across a field knowing the enemy will hear it. She screams *"onomatopoeia, Kevin! onomatopoeia!"* What she is really doing is reminding Kevin of when **he** tried to stump **her**, and what they were talking about then, because it is crucial to her saving him. Only Kevin can figure this out, so it makes no difference if she shouts it or if the villain knows the definition.
No.

Use the words that you want/need to use.

Kids have NO idea what is or is not a big word. Kids are a blank slate. Every word is big to them. So no word is big to them.

This is why there are plenty of 4 year olds who can rattle off the scientific names of dozens of dinosaurs.

Big words are not a problem for kids. To you it is a big word. To kids it is just a word.

Use the words you want to use. And use them appropriately.