163. For private pilot operations, a First-Class Medical Certificate issued to a 23-year-old pilot on October 21, this year, will expire at midnight on

Title 14 § 61.23 Medical certificates: Requirement and duration.

(d) Duration of a medical certificate. Use the following table to determine duration for each class of medical certificate:
...
(1) A first-class medical certificate
(iv) Under age 40 — 60th month after the month of the date of examination shown on the medical certificate.