I really do wish you the best of luck with your new venture, but at your age I think you are approaching this entirely wrong.
As the previous poster mentioned, you cannot enter into a contract, you will probably find it very difficult to get insurance too. I'm not sure what business requirements are needed to be met under your federal, state and local laws either, but I believe you would have major problems there too.
I reckon you have 2 options:
1) stick to pocket money work, let your customers know you don't have insurance etc. keep it simple and earn a few dollars. Whatever you do... don't try to pass yourself off as a legitimate contractor, this will only cause you, and possibly your parents problems in the future.
2) if you're one of these whiz kids who can make a million bucks before your 16'th birthday, then go for it!!! But get yourself set up properly and legally. This means your parents will need to be the official owners and operators of the business for legal reasons. The trouble is the insurance company probably still won't insure YOU to do the mowing work because of your age. If you go down this route you'll have to hire staff from day one.
Don't forget, the best training you'll receive is being a worker for a legitimate and respected contractor. A bit of work and training and you'll be ready to go on your 18th birthday with a far higher certainty of success.
Again, good luck and all the best.
Ramblings of an Australian lawn mowing contractor Lawn Care Tips