Many states allow a person age 18 or over to freely enter into contracts and other business arrangements. That means a teenager under the age of 18 may not. Therefore, if you are 18 or over and live in a state that considers the "age of majority" for contracts to be 18, you can move foward full steam ahead (unless, of course, your business involves the purchase and sale of alcohol or other items with an age restriction)! Just be sure to double-check with an attorney in your state that 18 is, in fact, the cut-off age for you.
The fall in new company starts does not necessarily mean that the American economy is less dynamic than it used to be, or that Americans are opening up new establishments at a lower rate than they did in the late 1970s.