Eligibility for a Remote Pilot Certificate with a small UAS rating is §107.61, read together with the alcohol and drug sections that can block an application. The list is short. People add a medical certificate to it. The regulation does not.
Age
You must be at least 16 years old. Eighteen is the private-pilot age and a common distractor. Fourteen is not the certificate age. The course's orientation, aligned with the FAA's testing rules, separates two moments: you may take the UAG knowledge test at 14, and you must be 16 to be eligible for the certificate. Section 107.67 requires the identification you present at the test to show a date of birth that meets, or will meet, the age requirement for the certificate before the airman knowledge test report expires. A 14-year-old can test. A 14-year-old cannot yet be issued the remote pilot certificate.
Do not mix this with aircraft registration. Part 48 says that if the owner is under 13, someone who is at least 13 registers the aircraft. That is an owner-age rule for the registration record. It is not the remote-pilot age.
English and physical condition
You must be able to read, speak, write, and understand English. If you cannot meet one of those because of a medical reason, the FAA may place operating limitations on the certificate that are necessary for safe operation. The baseline is still the English standard, not a waiver you grant yourself.
You must not know or have reason to know that you have a physical or mental condition that would interfere with the safe operation of a small unmanned aircraft system. That is a self-knowledge standard. It is not a requirement to hold a third-class medical, or any other FAA medical certificate. If an answer says the missing item in §107.61 is a medical certificate, that item is the one that does not belong. The condition standard and the English standard do belong.
Section 107.17 is the operating-rule version of that fitness standard. No person may manipulate the flight controls, act as remote pilot in command, act as visual observer, or be a direct participant if they know or have reason to know of a physical or mental condition that would interfere with safe operation. It still does not create a medical-certificate requirement. A condition that leaves you unable to see the aircraft, or a medication that impairs judgment, is a reason not to fly that day. It is not a reason to obtain a third-class medical before the next job.
Alcohol and drugs are separate. Section 107.27 applies §§91.17 and 91.19 to the remote pilot, the person manipulating the controls, and the visual observer. The operating limits include 8 hours from bottle to throttle, a 0.04 blood-alcohol limit, and no flying while using a drug that affects faculties contrary to safety. A conviction for the drug offenses listed in §107.57, or the act of flying under the influence even without a conviction, can be grounds to deny an application for up to a year or to suspend or revoke a certificate. That is enforcement, not a medical-exam requirement.
How you prove the knowledge
You demonstrate aeronautical knowledge in one of two ways. Pass the initial knowledge test covering §107.73. Or, if you hold a pilot certificate other than a student certificate issued under Part 61 and you meet the §61.56 flight review, complete training covering §107.74. The usual free initial course for that second path is ALC-451 on FAASafety.gov. You then apply through IACRA. TSA security vetting still happens. The Part 61 certificate does not skip it, and it does not already contain small UAS privileges.
After the certificate is issued, currency is a different rule. See recurrent training. Test-day logistics, including the identification you carry to PSI, are on the test day page.