Re: trying to remember what it was like for you back then: Yeah, that's an important piece. But it's not about you. It's about the story. Unless your story is something of a memoir, who you were back then doesn't really matter. What matters is the act of imagining yourself into the mind of the character you're creating.
I'm a girl. I was a nerdy, bookish girl. In high school, I actually took a book with me to parties in case I got bored. Which I usually did.
So, how do I write from the POV of an active, modern boy? Or a boy trapped in the body of a raccoon? Or an outgoing girl, for that matter?
Two things: Imagination. And observation.
Scope is so right. You have to find ways to be around kids, listen to kids, pay attention to how they think. I have an advantage, because as a teacher I've spent the last twenty years trying to figure out why the hell kids do the things they do.
(Sorry, sweetheart, we don't lick the furniture in first grade... Please put your shirt back on, honey... Yes, that's a very nice spider. Now maybe we should put it back where you found it?)
I remember watching a short film on Japanese anime master Hayao Miyazaki. He spent one entire afternoon at the train station, just sketching the way women's skirts moved when they walked. That's the kind of attention to detail that we need. When you really, really, really pay attention to the world around you, the voice gets easier. You can start to think like the kids you're trying to write for.