First, the question wouldn't cause silence to fall. As Dorium Maldovar explained in "The Wedding of River Song", "Silence must fall would be a better translation. The Silence are determined the question will never be answered."
Second, it's the "oldest question" because, as we discovered in "The Time of the Doctor", it's being asked throughout time & space, across the universes (plural), through the time cracks.
Third, it's important because it's being asked in Gallifreyan ("The Time of the Doctor"). It is, in fact, being asked by the Time Lords & a correct answer -- the Doctor's original name -- would identify Gallifrey's home universe. That would allow the Time Lords to bring Gallifrey back from the "pocket universe" in which the 13 incarnations of the Doctor hid it in "The Day of the Doctor".
It's this -- the return of Gallifrey -- that the Silence are so terrified of. They believe that it would re-ignite the Last Great Time War, with all the horrors that attended the war. That's why they're "determined the question will never be answered."
It's very hard to explain, as it appears in so many episodes and is so important. If you still don't understand, you should watch "The Time of the Doctor". "The Wedding of River Song" might be good as well.