The increased use of slang, shortcuts like txt msg, sometimes vulgarity use is higher with teen males though that can be dependant on dozens of variables.
Experience and knowledge -- which 99% of the time comes with age. Even if a teen is a genius, s/he will not have the social and emotional ranges that an adult has accumulated by the sheer passage of time (again this leaves out hermit types).
Read different styles angled toward YA and adult: I'm thinking Harry Potter 5,6,7 vs the Robert Ludlum's Bourne Identity/Ultimatum, or Patricia Cornwall's forensic novels with Chief Medical Examiner Kay Scarpetta vs the whodunits of the Hardy Boys.
Read what genres you love, and read the ones you don't like because even they can teach a new turn of phrase or how other writers deal with situations, characters, dialogue, settings, etc.
Watch shows and movies with good writers: Saving Grace, The Closer, Buffy the Vampire Slayer, Babylon 5, Farscape, old Twilight Zone classics are right up there. Actually many Tales from the Crypt are very well done, Alfred Hitchcock Presents reruns are great.
Anything 'reality' is garbage. The situations are contrived, the contestants/people are voted in to draw the most viewers by the greatest shock value and the most stupid/outrageous words/actions.
Being a good writer happens in part by being a good listener, you need to be able to discriminate between what you hear and what you're being told as in commercials.
Ex: Buy a new car for only $17,995 gets more people than a car for 5$ more at $18,000. What does the commercial push? Features, mileage, how a model is draped over the car (as if the woman is part of the extras), or how much more machismo aka mental Viagra comes from driving a Hummer vs a Subaru. Advertising sells what the advertiser wants you to believe, not what you actually need--which is reliable, inexpensive, transportation.
What do you want the reader to take away?
