Download a PDF filler application to your device, if necessary. Form I-9 is used for verifying the identity and employment authorization of individuals hired for employment in the United States.To complete Form I-9, you will need the latest version of Adobe Acrobat Reader or other PDF filler application. The minimum age for most full-time employment is 18, unless you are 17 and a high school graduate. The job posting will specify the age limit when it differs.