“ On and after July 1, 1932, no person rendering civilian service in any branch or service of the United States Government or the municipal government of the District of Columbia who shall have reached the retirement age prescribed for automatic separation from the service, applicable to such person, shall be continued in such service, notwithstanding any provision of law or regulation to the contrary: Provided, That the President may, by Executive Order, exempt from the provisions of this section any person when, in his judgment, the public interest so requires * * * ”