does "page boy" mean "Riedel has been to the Senate when he was nine?"
In the half-century since he had come to the Senate as a nine-year-old page boy Riedel had never before committed the breach of running out on the Senate floor. Now he raced up to Senator after Senator, spluttering, "The President has been shot—the President—he's been shot!"

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Yes, as strange as it sounds, it means that Riedel had worked as a page boy in the Senate when he was nine. Nowadays page boys (now just called pages) must be at least 16 years old.

Answer

Riedel had been IN/AT the Senate SINCE he was nine.

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