When I was 13, one of my bffs and I sold earrings we made at school for a business. Then we started selling them to a store at Faneuil Hall.

You can do it.
Keep your costs low and research your market for max profits.
Good luck!
:D

If you want a really great business idea. You might consider closet and kitchen organizing. Both are in high demand. Much simplier than interior decorating. You can get modular units at stores like Wal-Mart. And why do people hire it done? Because people are naturally pack rats and it takes another person to toss te unnnecessary clutter. Phone, flyers, an ad in a newspaper and you're in business. You can get te modular units at Wal-mart or another store and either have it delivered or your client pick it up (naturally tey pre-pay) Oh, and you'll need a Phillips and flat screw driver and adjustable cresent wrench for assembly.

But until you make $5000 or more a year, call it a "Hobby" instead of a business. You'll avoid the legal requirements and complexities that go with a business. And besides your parents won't lose you as a tax exemption prematurely.

When I was your age I was putting in pave stone and brick walk ways and patios as well as yard work. But I had no parents and supported myself and rented attic or basement space in classmates parent's homes from age 11. But I'm from a totally different era, when America was far freer and less complicated.

You will have to start the business with an adult over the age of 18. At 15 you cannot enter legal agreements such as business agreements, rental agreements, contracts or obtaining the business licenses and permits.

Well there are always people that have businesses younger than 18. But I can guarantee the business is not legal in any of the 50 States. Under age 18, it is still parents or guardians responsibility to file income tax for you. And of course easily lose claiming you as a dependent in the process. And it is the adult runing the business that will be sued should that happen. As said, a minor cannot be held legally responsible.

I'm 16 and I run my own henna business. Assuming you can't do henna, I'd reccomend cake/cookie/dessert making. Previously, I've sold custom-made cakes to my classmates and it worked out really well. I'd also tell you to work with a friend, so you can reach more people and share the responsibilities.
What I did was make tri-fold brochures with a little spiel about the business, say how part of the profits went to a good cause (Thai elephant reserve), and included an order form on the back. The cakes were nothing super-special; they were made from storebought cake mixes and homemade icing.
I gave people the option of white, chocolate, angel food, yellow, or funsprinkle cake, and then let then choose chocolate frosting, funsprinkle, or vanilla frosting in their choice of color. I left a space for them to fill in the words they wanted on the cake (color and style too), the shape of the cake, how many layers they wanted, the filling if there was 2 layers, sprinkle color and type, candles, when they wanted by, etc.

How about maybe something in fashion?

Get on with your education first and do business studies. Watch the dragons den and stuff to get ideas first.