Final answer:
A first-class medical certificate for pilots under 40 expires at the end of the 12th month after the examination.
Step-by-step explanation:
For pilots who are less than 40 years old, a first-class medical certificate expires at the end of the last day of the 12th month after the month of the date of examination. For example, if the medical examination was taken on January 15th, the certificate would expire on January 31st, the following year.