Qualifications
This is one of those cases where you're actually expected to go from the frying pan directly into the fire. For legal reasons, every state has certification rules―you're working with explosives, after all.
Let's use California as an example (source). First, you can't even think about touching so much as a sparkler professionally until you hit eighteen. After that, you're expected to join a crew.
In California, you'll spend two years honing your craft, during which time you'll work with several different crews and work at least eight shows in that time. Why several crews? Because you get more practice that way―and because you'll need not one, not two, but five letters of recommendation to even qualify to apply for your license.
Also, you have to wait until you're twenty-one to apply. Yeah, we know that's three years and we said two. It's California math.
Finally, after all that training, you can apply for your Pyrotechnicians License...but before then, you'll have to pass a background check, a safety and laws exam, and a drug test, get approval from the state Fire Marshall, acquire a separate commercial drivers' license to transport hazardous material (the fireworks), and throw the Ring of Power into Mount Doom. We're just kidding about that last one. They don't make you do that anymore.