Whatever your age, you can still benefit from supplementing with whey protein, especially during or immediately after a workout. I make protein shakes for my nephews when they visit, and they’re all under ten years old. Like someone else said, it’s just concentrated food.
As for other supplements: a quality multivitamin/multimineral is good insurance against micronutrient deficiencies in your diet. Fish oil is also beneficial at any age (get a good one - Biotest Flameout, or one that’s labeled “pharmaceutical grade”).
Creatine is helpful (again, it’s just a concentrated food extract), as long as you keep in mind that it’s not going to magically transform your physique.
I’d avoid most other supplements for a while, until you’ve gotten to a point where your progress or recovery has slowed and you feel you need some extra help.
But again, focus on food first; supplements aren’t going to make up for poor nutrition. Read Berardi’s Seven Habits article for starters, if you haven’t already: http://www.T-Nation.com/findArticle.do?article=291seven2
Yes, adults use net slang. And what makes you think you’re only going to run into adults here? You’re posting here, aren’t you? Anyway, you’ll find that the “adult world” is full of people of a wide range of maturities, whether online or in the workplace. Ignore the flames, just like you ignore stuff that happens in school.
And you’re old enough to learn to question everything you read and everything you’re told. Authorities are not always right and don’t always have your best interests in mind. Much of the time they’re just looking out for their own self-interest, or trying to cover their own asses - like putting “over 18” disclaimers on canisters of whey protein.
There’s a lot of money to be made in the supplement business, and plenty of unscrupulous people promoting substances of dubious worth. Be sceptical. But as far as stunting your growth, steroids or prohormones are about the only thing that would do that. When intelligently used, steroids are extremely effective and relatively safe drugs that have been unjustly demonized by the government and the press - but use of aromatizing steroids before your bones have stopped growing is a very very foolish thing to do. (For that reason, I personally feel that if there’s any place for drug testing in sports, it’s at the high school and college level, not the pros.)
Anyway, welcome to T-Nation, and good luck and happy lifting.