Various traditions claim that St. John was the youngest apostle, though the Bible doesn't give him an exact age.
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When looking at a painting of the twelve apostles, it is typically easy to find which one is St. John the Evangelist. Most painters have depicted St. John as beardless, looking considerably younger than everyone else.
Why is that?
The Bible does not mention the ages of any of the apostles. The only clue, according to the Catholic Encyclopedia, is the sequence of names given in the New Testament.
In the lists of the Apostles John has the second place (Acts 1:13), the third (Mark 3:17), and the fourth (Matthew 10:3; Luke 6:14), yet always after James with the exception of a few passages (Luke 8:51; 9:28 in the Greek text; Acts 1:13). From James being thus placed first, the conclusion is drawn that John was the younger of the two brothers.
Another reason why St. John is thought to be the youngest apostle during Jesus’ life is connected to the tradition that he died at an old age.
Various scholars have dated St. John’s death to between 98 and 100 AD.
While we don’t know how old St. John was when he died, it is thought he might have died between the ages of 80 – 100.
This is probably the most reasonable reason to believe that St. John was quite young during Jesus’ life. If he was already 80 years old in 33 AD, he would have died at the age of 147!
Technically, St. John probably had a beard, but artists preferred depicting him without a beard, to make him more easily identifiable.