I agree! Basic way to avoid dumbing down? Write as you would for adults. Teenagers aren't thick (well, most aren't anyway.). If you catch yourself thinking...
- Hang on, what if they don't get that?
- I'll have to leave out that subplot, because it's too complex.
- Maybe I should use a shorter word...
- I'll have to go into more detail, or they may not understand.
It is NOT good. Well, it is good if you think, no, that's fine. I will leave it. Think of it like this; if it's fine for you, then it's probably fine for a teenager.
Good luck!