There are several reasons why the Gospel of John was not likely written by an apostle. One, we know the apostles would have been uneducated, for the magnificent piece of literature that the Gospel of John is, it is extremely unlikely that it could have been written by a Palestinian peasant.
This John seems to have been of a priestly family, and was thus an erudite thinker. He was certainly not a farmer or a fisher.
Secondly, the Gospel has clear indications that it was written by an educated, scholarly Greek believer who had some resentment issues with the Jews. Notice that John refers to them constantly as “the Jews” distinguishing Jesus and his apostles from them as though they were not Jewish.
Actually, the word ‘Jew’ and its derivitives occurs only in non Latin translations.
The word commonly mistranslated as ‘Jew’, ‘Iudaeus’ means literally, ‘Judaean’, or native of Judaea. The word is used in the same context as Galilean or Samaritan, and is used purely as a description of nationality.
In the OT, however, the word is commonly used, before , usually, the inception of the nation of Judea, to refer to members of the clan of Judah.
The name used by all these people, in the time of the ministry, to refer to what some of us think of as Jews, was ‘Child of Israel’, or in short form, ‘Child’.
Thirdly, the Gospel of John dates as the final gospel. John is not a synoptic gospel and presents some very striking statements that are mysteriously missing within the Matthew, Luke and Mark, especially when considering their significance to Christian theology. At times it even contradicts the synoptics (for example Jesus’ execution is incited by the resurection of Lazarus as opposed to causing havoc in the Temple)
The Gospel of John was likely composed around 100 CE. Jesus died around 30 CE. In these times, especially for peasants, the lifespan was considerably short. Lets say that John was 16 years old when he became a disciple of Christ. By the time this gospel was written he would have been between, say, 70 and 86 years old. It is unlikely he would have lived that long.
It is commonly postulated that the Gospels were produced in a very short effort. It is never considered that what we hold is the product of a lifetime’s continuous editing.
Thus we see in John, some sprkling jottings of the instant, and also some deep meditations that have taken years to crystalize.
When analyzing this from a historical perspective, it is extremely unlikely that Johannine authorship was of apostolic origin. However, from a faith perspective (ie. God gave the disciples enhanced literary skills and extended life) then it is not out of the question.
Bear in mind that not a few spiritual souls did in fact live to a great age, while maintaining their faculties. The age of 120 years was not so uncommon. 900 years as with Methusalah though seems to have been apochryphal.