The topic says it all. Please keep this about general truths and not specific advice that only applies to you.
My ugly truths:
Math gets a lot harder. Enjoy the arithmetic while you can.
The government and the police are not your friends.
Appearances matter, even to people who should know better.
What are yours?
I was trying to think of something appropriate, but then it hit me. I was a happy ten-year-old because I was so completely clueless.
If I see my ten-year-old self, I’m going to tell him to keep playing Pac-Man and collecting G.I. Joe (which are action figures, dammit, not dolls).
Ignorance is bliss, you know?
As you get older, you will find out that much of society isn’t like it appears right now not even for some of your family and dear friends. Lots of people have deep-dark secrets and engage in secret behavior. A lot of those happily married aren’t. Spouses cheat on each other more often than you would believe and if you happen to be in the right environment, you will find out about it. Lots of people have drug and alcohol problems that they keep well hidden. It could be your favorite teacher or a dear relative. Some of them may be sexual predators, completely closeted homosexuals with spouse and kids, or into something else that you would never guess. Some of your professional colleagues will be fired and maybe even arrested for embezzling.
The key to this phenomenon is that you will never be able to reliably match up which transgression or problem goes with which person even if they are among your closed friends. You have to be aware that shocking things will just pop up out of the blue sometimes and you can never truly trust the vast majority of people in all ways.
You have a good point. For the sake of entertainment, let’s ignore that fact or pretend we hate our 10yo selves and want to ruin their innocence. (come to think of it, that’d make an interesting thread of its own).
PS: They’re dolls.
You’re going to move in a couple of months and your mother’s going to throw out your comic books and baseball cards.
Don’t let her touch “The Death of Superman.”
1.) That dude ain’t your dad. Your Mom banged his best friend.
2.) Start looking for something you want to be when you grow up and study it, hard.
3.) Smile you sullen little bastard!
Sex makes you act stupid. Try to maintain some dignity in a few years, please.
I hate to break it to you, but the adults do not have it together. It’s an act. They’re pretending. And it’s not a “meritocracy of the nice people” out there. The pushy authoritarian people who should be “getting theirs” are as often as not in charge of things.
Ten years old, eh? Grade six.
:: shudder ::
Things will get worse for the next two years. All that bullying and teasing? They’re going to get a lot bigger and scarier until you get to high school. And even then, grade nine won’t be a picnic. It’s something about being that age. Kids that age are evil.
You need to learn social skills. This is even more important than math or art or English.
You need to exercise and learn how to use your body. If you don’t, it will cause you to miss goals and underachieve and screw you up for the rest of your life. There is a point to gym; it’s not just shame and humiliation and Being Picked Last. And… for the love of god, take the gymnastics and martial-arts courses. But learn about exercise and stretching as well.
You are not as ugly and unattractive as you think you are. But if you don’t learn social skills, you’ll never see the signals that might tell you someone is interested in you. You’ll never learn differently. And you’ll go thinking you are ugly and unattractive until you are old.
Take typing, even if you think it’s a ‘girly’ course. In a couple of years, that’ll be a good thing. (And there’ll be this thing called the ‘personal computer’…)
Trust your inner art and muse. It’s the surest guide for you. All else is, in the long run, a false course.
Life isn’t fair. It just isn’t. The nice guy doesn’t always win, but you can manage to do fairly well anyways. Try not to let it get to you that pretty, stupid people can manage to go real far without any effort. And try to avoid manipulative people.
You know that fleeting, imaginary picture you had of being a grownup when you were seven or so? Of being all alone in a tiny little room, eating chicken soup every day? You have no idea how prophetic that was.
When you grow up, Pluto is not a planet. Them’s the breaks, kid.
DO NOT give away your She-Ra dolls and Crystal Castle!
Some people are nasty. They’re mean and hateful and vindictive and they’ll say hurtful things just to make you cry. This applies to adults, too, not just the kids on the playground.
At some point, your parents are going to go through a second childhood. If you’re lucky, they’ll wait until you’re grown to do it.
The Authorities - the police, the teachers, doctors, nurses, parents - are not always right. It’s okay to question what they say.
Sex isn’t as great as everybody says it is.
Becoming A Woman (aka Getting Your Period) is only exciting for maybe 12 hours. Then you’ll realize that you’re going to have to go through this every month for the next 40 years and that’s when the mood swings and bitchiness kicks in.
Don’t date men with homemade tattoos. Just…don’t.
Going to the dentist is scary, no doubt about it. And it hurts (more then than now). Still, be a big boy and go do it. Otherwise, you’ll have crooked teeth and like it or not, people will think you look funny.
Hey, and here’s maybe a little easy advice. When you’re throwing a baseball or football, throw across your body, not straight out from your body. You’ve been doing it like you’d think would make sense, but that’s why you haven’t been able to throw very far. (Never had a coach explain this to me.)
Stop eating so much, fatass.
The school system you’re in isn’t very good.
Several years ago, I composed an OP called “Bitter truths you learned growing up” which answered this very question, but for whatever reason I never posted it (although I was sufficiently impressed with it to store it in some musty corner of my hard drive for all this time). I might as well post it here, or else it will never see the light of day…
Keep in mind I was a much darker, more cynical person back then 
:
Bitter Truths I Learned Growing Up
Aside from your immediate family and close friends, nobody gives a rat’s ass about you.
You really aren’t the smartest, most talented, most beautiful, or most interesting person in the world. Sorry.
The best days of your life are over.
Becoming president of the U.S. is a whole hell of a lot harder than your parents made it out to be (unless you were born into the right family, of course).
Level of intelligence has little or nothing to do with how much you get paid.
People really do judge you by your looks.
There are way too many painters, cartoonists, novelists, movie directors, screenwriters, playwrights, actors, singers, and composers in the world. Artistic creativity is virtually worthless if you want to make a living.
All those confusing things that make modern life so complex – credit cards, loans, mortgages, insurance, taxes, inflation, interest rates – have a single purpose, which is to screw you out of your money.
Money is an illusion. The value of currency is based on your faith that the government will exchange it for something of real value (which they cannot and will not do).
Both the Democrats and the Republicans are lying, cheating bastards.
Some of the vilest acts are committed by people who believe they are doing the right thing.
Even the finest human beings on the planet have flaws. Lots of them.
Most people are perverts, although some hide it quite well.
Being against the law doesn’t necessarily make something wrong, and being legal doesn’t automatically make it right.
Alcohol is a drug. Just like heroin or cocaine. When people say they’re gonna grab a few beers, it’s code for “I’m gonna go get high on ethyl hydrate”.
All people have the capacity for evil, not just an imagined subclass we call “criminals”.
There’s no such thing as Santa Claus, the Tooth Fairy, or the Easter Bunny. There are no ghosts, UFOs, angels, or demons, and there is no God or Devil. There is no afterlife, and when you die, it’s lights out.
The truth sucks (see #17). You can either learn to accept it or go into denial like 90% of the population. It really doesn’t make much difference in the big picture.
You will never find your Prince Charming or your Cinderella. If you’re lucky, you’ll find a person whose flaws you can tolerate for longer than a few years.
Childbirth is not a miracle. Any woman with a functioning womb can do it, provided she can find a man willing to donate some sperm (not all that hard to do). There are too many goddamn childbirths on this planet.
The number one factor contributing to success is not intelligence, talent, courage, honor, charisma, or even ambition. It’s dumb luck.
No matter how badly you want something, the minute you actually get it you’ll forget about it and move on to wanting something else.
As an adult, you’re exactly like you were as a kid, except you have less energy, enthusiasm, confidence, and hope.
The English language is the most difficult language on the planet. Even native speakers, after years of intensive study, can’t get it right. Whoever invented it was obviously insane.
Even sex, drugs, and rock ‘n’ roll get boring after awhile.
Money can’t buy you love or happiness. It can buy you success, power, status, influence, respect, glamor, luxury, freedom, security, and recognition.
Animals are the greatest oppressed underclass on the planet. Millions of them are routinely humiliated, killed, exterminated, maimed and tortured, and nobody seems to care about it except those PETA lunatics.
Meat is produced from the slaughter of innocent creatures. But it tastes so damn good!
A majority of the categories we impose on the natural world and ourselves are meaningless, since most things exist in gradations and subtle shades of gray that cannot be easily pigeonholed. Consequently, most of life’s important questions do not have concrete or simple answers. This really pisses people off.
An overwhelming majority of the world’s murders, assaults, rapes, and genocides are fueled by a pesky little hormone called “testosterone”.
Women are moody, hysterical, petty, feckless, and vain. And they’re the better sex.
Stereotypes are false. Sometimes.
A lot of people are mean and stupid. A lot of people are intelligent and kind. Guess who gets more attention?
If you want something done right, chances are you have to do it yourself.
If you want to make it to the top, you have step on a lot of toes and kick some serious ass. That’s why most politicians, military leaders, and corporate executives are dickheads.
All human emotions – sadness, desire, love, anger, hatred, fear, and hope – are merely the result of brain chemicals and can be manipulated at will by the right combinations of drugs.
Even if the human race manages to survive nuclear war, famine, disease, overpopulation, natural disaster, climate change, and greedy politicians, the dying sun will eventually turn our planet into a lifeless, smoldering cinder. And if we somehow survive that, the universe will either implode into an infinitely small singularity or expand forever into a lifeless, entropic soup. Any way you cut it, we’re screwed.
Have a nice day. 
Being one of the smartest in your small town class doesn’t allow you to drift thru life with ease. You have to work hard also.
That “pot” thing you’ve heard of? It doesn’t help with that drifting thing either.
Women are not the fairer sex. They’re just as cheating, selfish, lieing, bastards as men are. Give their opinion no more creedance than you would anybody else’s.
In ten years time you will be at a wedding and Carol is obviously flirting with you, have sex with her.
Ease up on the weed freshman year of college
No one makes parts for a 71 rotary Mazda in 1985, don’t buy it
It’s not your fault, dad really is a psychotic