Your Character's Voice
This has oodles of variables from age, sex, education, areas lived, social status, family income, friends and acquaintances, personal influences, and physiology of the mouth, lungs, vocal cords, etc.
For modern teen males, the increased use of slang and shortcuts like txt msg are popular. Sometimes vulgarity use is higher with teen males though that can be dependant on dozens of correlations.
Experience and knowledge--which 99% of the time comes with age will make a difference in the voice. 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).
Traumatic events can also shape a characters voice and lock them into a replay of a certain past that will show in times of stress.
YA voice vs adult, think JK Rowling's Harry Potter 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 the 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. Some of my favorites are: 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 and Alfred Hitchcock Presents suspense classics are great.
It's not just good writing, it's how the character says it, body language, pitch, emotional input. Communication is only 5% verbal, the rest is how a person looks at others or the surroundings, how the body is held tense or loose, what the hands are doing, shrugs, grimaces, sniffs, fiddling with exterior objects, and more.
Reality TV is useless garbage. Survivor, The Great Race, cook-offs, designer fetes, singing contests--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 also known as mental Viagra comes from driving a Hummer vs a Subaru.
I worked in advertising and what advertising sells what the advertiser wants you to believe, not what you actually need--which is reliable, inexpensive, transportation. Be an educated observer and question what you hear. Crest toothpaste won't make you popular, the Armani clothes won't make you a magnet for love, the Infiniti sedan won't enhance your business, Burger king fast food won't increase joyous times with friends, the Nike sneakers won't make you a better athlete.
What do you want the reader to take away?