Final answer:
The duration of First Class Medical Certificate privileges for a pilot under 40 is 12 months, after which it must be renewed to maintain privileges.
Step-by-step explanation:
The duration of First Class Medical Certificate privileges for a pilot who is under 40 years old is 12 months. After this period, the certificate must be renewed for the pilot to maintain the privileges associated with First Class medical certification. It's important for pilots to keep track of their medical certificate's expiration to ensure they remain in compliance with the regulations set forth by their regulatory body, such as the Federal Aviation Administration (FAA) in the United States.