Dear Stacey at 15,

Hey you! Yes, you there, the one putting a hole in the ozone layer with all that Aqua Net you're spraying in an effort to make your bangs stand up straight! It's me--you at 40. Boo!
How are you doing? You seem a little angst-ridden. Why don't you turn off that music? Yes, I know the Violent Femmes rock, but they're awfully depressing, and since you will eventually be diagnosed with clinical depression, they certainly can't be helping. You know what you like to listen to now as you're be-bopping down the highway? That's right, Duran Duran. THAT's how cool you're gonna be at 40.
Right now, you're probably thinking about how you're going to meet Megan in the girls' bathroom for a cigarette before your first class. Guess who you don't speak to at all anymore? That's right, Megan. We don't even know what state she lives in. Stop worrying so much about being BFFs. And see this turkey neck? Those cigarettes gave us this. Quit now!
What's that, you ask? How could we have possibly lost touch with Meegs? Take a careful look at your friends. We don't see any of them anymore, except on Facebook, which I don't even want to explain to you right now. You know who you do get together with a couple times a year? Alicia and Laura. That's about it. People grow up and grow apart. Stop worrying so much about your friends' dramas and worry more about getting that chemistry grade up.
The good news is you'll never forget your English teacher right now, Ms. Lacosse. Be nicer to her--she will have a huge influence on your career path. And you know your secret dream to own a bookstore and read all day? We get to do that! Except for the reading all day part. Businesses take a lot of work, you know.
Hear your mother out in the kitchen, telling you to get a move on before you miss your bus? Know how everyone always says you two don't look anything alike? That'll change. You're pretty much her blonde twin now. That's right--you, too, will soon carry a big purse and wear sensible shoes. You won't care at all about how silly you look. And no, sadly, Converse All-Stars do not constitute sensible shoes. But you will occasionally throw some arch supports into your Cons and wear them for nostalgia's sake. We haven't totally changed, you know.
You and Mom are even friends now, can you believe it? Being an adult isn't so bad. You know who your best friend in the whole world is? That's right, your big sister. That hasn't changed. Awesome, right?
You should be nicer to your aunts, by the way. Three of them will be rather influential role models for you as you get older. What's that? Why did I say three, and not four? You're going to lose one when she's fairly young. I'm not going to tell you which one. That way, you can be more appreciative of the four you have now.
Why are you sighing and rolling your eyes at me? What do you mean, I just don't understand you? I WAS you, stupid. Let's get a few things straight: first of all, you're not fat. One day, you're going to make it your personal weight loss goal to stay under 170 pounds. That's about 40 pounds heavier than you are now. Shut up and put on a bikini. I do wish we'd worn those more when we could.
Second of all, know that guy who you get all crazy and giggly around when you pass him in the hallway? Yeah, we don't even remember his name anymore. He's not the love of your life. In fact, pretty much everyone you've ever dated isn't the love of your life or even good enough for you. You'll meet a guy your senior year that actually knows who Roald Dahl is. Date him. Date that guy.
I know right now you're thinking that nothing could ever be more important than whether or not Amie's pregnant, or if whatshisname will notice the cool tie-dye you're wearing today, or when you will finally be able to escape the farm and live on your own. You're so wrong. Here are the answers: Amie's not pregnant; nope, whatshisname doesn't know or care that you're alive; and someday, you'll miss both the farm and not having to worry about a mortgage. 
The most important people in your life now are your family, a guy you won't meet for another couple of decades, and a handful of friends you haven't even met yet. And some kids who haven't been born yet. No, not ours. We haven't completely lost our minds. But you do have a sister and will have in-laws that will be inclined to reproduce. It won't be as terrible as you think. You'll actually like your nephews and niece. Because you're going to be the coolest aunt ever.
Take another look around your room. Take note of that poster of Nick Rhodes rolled up in the corner; those Jack Ketchum and Stephen King books on your shelves; that old snapshot of Gordie Howe you have taped to your mirror. Good news: you're going to meet every one of those people at some point in your life. Yes, even Nick Rhodes. I'll allow you a little teenage scream for a moment. And know how you're a closet wrestling fan? You're going to have the best time hanging out with Jake "the Snake" Roberts when you're thirty-nine. I know. Sooo cool.
I wish I could tell you more, but it's time for me to go. There's so much more I want to tell you--like which college you should pick, and which marriage proposal to turn down, and a million other things, but I know you still need to make those choices and mistakes on your own.  Good luck. You're not alone.
Oh, and if you could play these Powerball numbers on February 14, 2007, that would be helpful: 
35 01 15 37 45 32 3

Love,
You at 40.