How old do legally have to be to start your own business?

Tricky question. You can be in business to throw papers at an early age. You can also cut yards. But, to own a legal business outright and in your own name only that has a Tax ID number and or is incorporated or has a filed DBA you must be of a legal age of 21. Which means that you can enter into a legal contract with any potential clients and or suppliers and be held personally responsible.