Given the task definition and input, reply with output. In this task, you're given a passage, further information available on a particular linked term from the statement, and an answer term. Your job is to generate a question that can use the information provided to obtain the given answer. You should use the information on both passage and link information to create the question. Note that the answer to the question should be exactly the given answer, and if the answer is none, the answer to the question shouldn't be obtainable from the passage or linked information.

Passage: The earliest surviving written criticisms of Islam are to be found in the writings of Christians who came under the early dominion of the Islamic Caliphate. One such Christian was John of Damascus (c. 676749 AD), who was familiar with Islam and Arabic. The second chapter of his book, The Fount of Wisdom, titled "Concerning Heresies", presents a series of discussions between Christians and Muslims. John claimed an Arian monk (whom he did not know was Bahira) influenced Muhammad and viewed the Islamic doctrines as nothing more than a hodgepodge culled from the Bible. Writing on Islam's claim of Abrahamic ancestry, John explained that the Arabs were called "Saracens" (Greek , Sarakenoi) because they were "empty" (, kenos, in Greek) "of Sarah". They were called "Hagarenes" because they were "the descendants of the slave-girl Hagar".
Link Information: none Answer: none
How old was John of Damascus when the Islamic Caliphate overtook the land of his home?