Let me revise my previous post. John was probably in his early twenties when he became an Apostle.
According to Saint Jerome, John died in the 68th year after the year of Christ’s Passion. According to Saint Epiphanius, John was 94 years old at the time of his death. If so, then John was about 26 years old at the time of Christ’s Passion (68 years earlier), and was about 23 when he became an Apostle. This fits the description of John in the Gospels, which portray him as the youngest of the Apostles. It is difficult to confirm Saint Epiphanius’ assertion the John lived to the age of 94. However, John could not have died much younger than the age of 94. If John had died even at the age of 89, he would have been barely 21 years old at the time of Christ’s Passion and would have been only 18 when chosen as an Apostle. If John had died at the age of 84, he would have been only 16 years old when Christ died and only 13 when chosen as an Apostle. For this reason, the assertion that John died in his mid-nineties must be correct, at least to a good approximation.