You don't need to be an adult, or someone living in the US, to own a copyright. As for "use," maybe you're referring to the age of majority, which can regulate someone's competency to enter into a contract or license, for examples. Minors are able to "disaffirm" a contract.
The age of majority does vary by state, and in FL, you have to be 18 to be considered an adult.
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Federal law governs use of copyrights. However, state law may sometimes come into play concerning whether minors can license or assign copyrights in their works to others. You should consult with a Florida copyright law expert on this issue, because it may require understanding of both Florida contract law as it relates to minors and copyright law. As a practical matter, anyone who wants to license a work created by a minor should get permission of both the minor and her parents.
A similar question was pose recently and received a number of responses -- with different conclusions. Visit the link below [and read the comments] and then speak with your own copyright attorney.
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.