Probably one of the easiest ways to write to a target audience is to spend time with that target audience. Some of us don’t have that luxury, but you can find ways to be around kids or teens without interfering with their activities or looking like a creeper.
If you have children, I don’t recommend stalking them. You can certainly interview them to see how they would act in a certain situation or what they are learning at school, but they’ll feel threatened if you watch them all the time to see how they act. They won’t act normal. You could host sleep-overs and parties to see how your kids act with their friends in that setting.
If you are a teacher, being on cafeteria duty, recess duty, bus duty, or car-rider duty can have its advantages. You are in a prime position to watch how students interact with one another outside of the classroom, how they talk to their friends, how they settle differences on their own, etc. For teens, you could be a teacher sponsor for a student group like the homecoming decorating committee, computer club, cheerleading, etc. You could offer to chaperone after school activities like dances, or offer to work the concession stand at basketball games. You would be surprised how much uncensored contact you can get with teens in school-related activities.
If you aren’t a teacher, but have time to spare, offer your services as a substitute teacher. You only need 30 hours of college credit (as far as I know).
If you are a member of a church, volunteer to be a Sunday school teacher. Host church-related activities for kids like discussion groups, church league sports, or special get-togethers once a month.
The further away you are from your target age, the harder it is to write for them without sounding motherly or preachy. Ten year olds now don’t do the same thing they did fifty years ago, or even fifteen years ago. It’s been ten years since I was a fifth grader, and I’ve already forgotten about DH, computer lab, hall passes, the pressure of school dances, etc. Fifth graders don’t do the same things I did when I was that age. They don’t have Gameboys; they have Nintendo DS and PSP. They don’t trade Pokémon cards; they play with Tech Decks and trade Silly Bandz.
If you don’t have access to your age group, and you have no way to get access, then you need to do research. Watch TV shows about middle-schoolers or teenagers. You’d be surprised how accurate they are. I’ve been watching Ned’s Declassified School Survival Guide to get a better idea of middle school as it was five years ago (and I really like that show). For preschoolers, watch Nick Jr. and PBS – shows like The Backyardigans, Blue’s Clues, Sesame Street, The Little Einsteins, Dora the Explorer, etc. Watch a lot of Nickelodeon, Cartoon Network, and Disney to get an idea what middle graders are interested in - shows like Spongebob Squarepants, Wizards of Waverly Place, Fairly Oddparents, Ben 10, iCarly, etc. For older teenagers, watch more primetime television like ABC, CBS, NBC, all those acronym channels - shows like Grey’s Anatomy, One Tree Hill, True Blood, Desperate Housewives, Smallville, Degrassi, etc. Teens watch pretty much everything adults watch.
I find that TV is better suited for research than reading books from your age group (though I also recommend doing that too) because TV is more current. Maltida is an awesome book for beginner readers, but it was also published 22 years ago. Harry Potter and the Sorcerer’s Stone was published 13 years ago. They both take place in other-worldly settings, so they can get away with being a bit dated. They’re essentially timeless. Books that take place in a school setting or a modern setting have to be nailed to what is going on today. Like I said before, school now is not like it was in the ‘80s or even the ‘90s. Kids learn new subjects, have more options in school, have different slang words, and text their friends in the middle of class instead of writing notes.
To write to an age group, you have to know a lot about that age group. Do what you can to be around your age group, and if you can’t accomplish that, do research. Lots and lots of research.