How old was John on the Isle of Patmos?

There is no record of the birth of John, and there is some debate as to whether or not John of Patmos was the same John who was Jesus's disciple. According to historians the Book of Revelation was written sometime between 68 and 96 AD; so if John of Patmos was the same person as John the Apostle then he would have been well over 60, possibly even in his 70's.