Christian tradition holds that John did live to be 80 or 90.
We know from Polycarp, that John was still active in Ephesus, and baptised him directly. Following Schaff:
It is safe, then, to say that the apostle John, with other disciples of Christ, came from Palestine to Asia Minor. If Polycarp, on the day of his death (Feb. 23, 155), was looking back on eighty-six years of life as a Christian, not as a man, and was thus baptized in 69, and if his conversion (according to Iren�us, Haer., III., iii. 4) was the work of an apostle, this migration to Asia Minor must have occurred before that date, possibly as a result of the outbreak of the Jewish war. John, then perhaps not more than sixty or sixty-five, would thus have been able to devote some thirty years to the fostering of Christian life in the province.]
Likewise, of Revelation, Schaff writes:
Space forbids going into the long history of the hypotheses which have been set forth as to the growth of the book, which is frequently held to have been a lengthy process. The following conclusions, however, seem safe. The assertion of Irenaeus (Haer., V., xxx, 3) that the visions were seen and the book written toward the end of the reign of Domitian, or about 95, finds support in the numerous historical data of the opening chapters. The designed and immediately accomplished introduction of the book into public liturgical use precludes the possibility of any notable alterations in it between 100 and 150. The author, as his name and idiom show, is of Hebrew birth, and about 95 had a recognized position of authority over the church of the province, without having any contemporary rival of the same name. He is the only John of Ephesus of whom anything is known from a tradition reaching back into his lifetime and in decisive points independent of his own writings. That he does not call himself an apostle is no proof that he was not one; his apostleship had no immediate connection with his apocalyptic purpose, and he does not describe himself at all
Put more simply, whoever wrote Revelation was the most important John out there, and that he was the same disciple makes sense.
He was, according to tradition both the youngest of the disciples and the one who lived the longest. Tradition also states that the older he grew, the shorter his sermons would be, until they could simply be "Little Children, Love one another!"
The average life expectancy is much lower, because so many people died as young infants or children - but once you made it to you teens, you could expect to live quite a while.