The Bible does not specifically give the ages of any of the original twelve disciples. However, a few observations can be made regarding their ages.
First, Scripture teaches Jesus was about 30 years old when He began His public ministry (Luke 3:23). In Jewish culture, disciples (or students) were generally younger than their teacher. Therefore, it is likely the disciples were under 30 years of age. Jesus also referred to them as “little children,” possibly indicating they were several years younger than He.
Second, many of the disciples worked as fishermen. James and John specifically left their father in the boat to follow Jesus (Matthew 4:21–22). This means they were old enough to work full-time. They were likely at least teenagers by this time, since they could leave home to follow a rabbi.
Third, Peter is noted as already married when he began following Jesus. His sick mother-in-law is mentioned in Matthew 8:14. This means Peter was at least old enough to be married; at the very least, Peter was in his mid-teens.
Fourth, the later lives of many of the disciples help to reveal their possible ages. John lived the longest, passing away in the last decade of the first century, according to church history. This was 60 years after walking with Jesus. Even if John were in his 90s when he died, he would have been no more than 30 years old at the time he was with Jesus. Since John was old enough to care for the mother of Jesus (John 19:26–27), he was probably at least 20 at the time of the crucifixion.
Peter referred to himself as an old man when he was in his 60s, about 30 years after walking with Jesus. This may mean Peter was in his 20s or 30s at the time he was with Jesus. The Gospel of Matthew was written 30–40 years after Jesus’ resurrection, indicating that Matthew was perhaps in his 20s when following Jesus on earth.
Jewish culture made it customary for a child to begin his religious training at the age of 5 and to continue to age 12 or 13. If a boy was intelligent and interested in continuing his religious studies, he would then seek a rabbi to disciple him and would follow and pattern his life after the rabbi until age 30. At that time he could take on disciples of his own. A young man’s discipleship training under a rabbi would usually begin between the ages of 13 and 15. If this pattern was consistent with the followers of Jesus, some of them may have joined Jesus as early as age 13 and would have still been teenagers at the time of His death, resurrection, and ascension.
Though the Bible does not give the exact ages of the disciples of Jesus, it is likely they were all between the ages of 13 and 30 at the time they followed Jesus. This view allows for some variety in their ages, with John likely the youngest and Peter perhaps one of the oldest since he was already married.