The Epistle From Patmos
Of all of the twelve apostles, only John survived to old age (which doesn't mean that he wasn't martyred then; see Martyrs). John's long life wasn't just some sort of favoritism, it was a matter of him being given the responsibilities for things that required him to remain alive. The first was to care for Mary, the mother of Jesus, through to the end of her natural life:
"19:26 When Jesus therefore saw his mother, and the disciple standing by, whom he loved, he saith unto his mother, Woman, behold thy son! 19:27 Then saith he to the disciple, Behold thy mother! And from that hour that disciple took her unto his own home.
"Write in a book and send it unto the seven churches which are in Asia"
The age difference between Mary and her nephew John (see Aunt Mary?) was likely less than twenty years. Since we know that the book of Revelation was written around 90 AD, about sixty years after Christ's crucifixion, Mary likely lived to be quite old. There was no other reason to protect John for that long; he could have been given to write the apocalypse (see the Fact Finder question below to understand the meaning of apocalypse and revelation) anytime sooner.
When the time came that John was held as a prisoner by the Romans (indicating that John was nearing the end of his ministry and perhaps his execution) on the island of Patmos (located off the west coast of Turkey - see the map), John was given to write Revelation, which, although regarded as a book of Prophecy (which it is), was originally an epistle (most of the other Epistles also were composed of prophecy), a book-length letter, to the "seven churches of Asia," given to John by a vision of Christ.
"1:9 I John, who also am your brother, and companion in tribulation [see Trials and Tribulations], and in the kingdom and patience of Jesus Christ, was in the isle that is called Patmos, for the word of God, and for the testimony of Jesus Christ. 1:10 I was in the Spirit on the Lord's day, and heard behind me a great voice, as of a trumpet, 1:11 Saying, I am Alpha and Omega, the first and the last: and, What thou seest, write in a book, and send it unto the seven churches which are in Asia; unto Ephesus, and unto Smyrna, and unto Pergamos, and unto Thyatira, and unto Sardis, and unto Philadelphia, and unto Laodicea.
Fact Finder: Why is the book of Revelation also known as the Apocalypse?