Neat concept!
I am currently seventeen years old. To someone twelve years and nine months old:
*Focus on the positive in every situation that you possibly can. Yes, it is important to be aware of the negative aspects for reasons of safety and practicality, but also the majority of your mental and emotional energy to be allocated to the positive aspects. People are batshit crazy, yes, but they also have traits about them that can make life a truly remarkable experience.
*BALANCE is key. Work diligently and live responsibly, but allow yourself to enjoy the pleasures of life.
*A healthy diet can make all of the difference in the world. Stay away from high fructose corn syrup, monosodium glutamate (MSG), and aspartame. This is not so much to do with weight issues as it is to do with general, overall health.
*The above being said, it is perfectly okay to "cheat" every once in a while; if you REALLY want that piece of cheesecake after a stressful week, then go ahead and indulge: one piece of cheesecake is not going to harm you in the long run.
*Find love wherever you possibly can; whether it be people, nature, music, art, logic, math, science, or washing machines, love SOMETHING. Love is vital to the core of our beings.
Yes, this sounds a lot like advice that would be given to adults. I see no reason, however, to withhold such information from a teenager or preteen. The sooner they realize such things, the happier they will become in the larger scope of time.