“Celebrity” is not an official title but a relative term used to describe any person that you may hire to promote your products or services. The person does not have to be famous nationwide and can be just somebody well-known in your local community or the internet.
Yes, if you want to hire a minor to promote your company's product or services, you should sign a Celebrity Endorsement Agreement, but not necessarily with the minor. Most states do not legally recognize a minor’s contractual obligations, so you will actually be entering into the agreement with the minor’s parent or legal guardian.
The terms of the Celebrity Endorsement Agreement depend on your industry and the campaign that you have in mind. The terms can be custom, but they usually include things like appearing in public or at the company's events, talking about your product in public, and publishing photos of your product on social media.