Correction: either is a hundred years old or is one hundred years old
I'm guessing that the first suggested correction is the correct one. Either way, I seriously doubt that organizer Rory Richards said that the pavilion "is a one hundred years old". I once learned that when writing numbers in a published work, one should write out the numbers - as in, one instead of 1. A professional writer should know that "a 100" instead of "a hundred" is just plain wrong.