For anyone who thinks this is a merely academic question, you can learn about the innovative work of teen entrepreneurs here, here, or here. In my years of reviewing applications at the USPTO, this issue came up more often than you'd guess.
The vast majority of USPTO-related questions can be answered by federal law. But, unusually, the answer to this question turns on state law. Specifically: can the minor “validly enter into binding legal obligations, and . . . sue or be sued, in the state in which he or she is domiciled”? TMEP §803.01. If so, then they can file the application in their own name. If not, they’ll need a little help from a parent or legal guardian. In that instance, technically, the applicant is identified as:
[Name of parent/legal guardian], [country of their citizenship] citizen, parent/legal guardian of [name of minor]
Clunky? Yes. Necessary? Also, yes.
In good news, if the minor comes of age before review of the application is complete, you can amend the application to change the applicant’s name without having to file an assignment (and without having to pay the corresponding fee to file an assignment).
This information was posted on January 26, 2021 and was accurate as of the date of writing. However, the law changes frequently, and readers should not rely solely on general online information but instead should consult a licensed attorney by asking questions about their specific issues when they need legal advice.