shall be separated from the Service on the last day of the month in which such individual under subparagraph (A) or such Foreign Service criminal investigator/inspector under subparagraph (B) attains 57 years of age or completes 20 years of service if then over that age. If the head of the agency judges that the public interest so requires, that agency head may exempt such an employee from automatic separation under this subsection until that employee attains 60 years of age. The employing office shall notify the employee in writing of the date of separation at least 60 days before that date. Action to separate the employee is not effective without the consent of the employee, until the last day of the month in which the 60-day notice expires.