How old were the disciples of Jesus when they joined him?

The Bible does not say explicitly, but there are a few hints that at least some of them were relatively young — probably teenagers or in their early 20s.

In Jewish tradition, a young man began following a Rabbi between the ages of 12 and 30, and usually when he was less than 20. So that would make most of the apostles teenagers when Jesus called them to follow him.

Another relevant fact is that John lived until at least AD96 when Revelation was written, which is 66 years after Jesus died. That suggests that John was almost certainly a teenager when he joined Jesus.

Matthew 17:24-27 may provide further evidence. Here, Jesus and Peter pay the temple tax which was required to be paid by every man aged 20 years and older (Exodus 30:13-14). The other disciples were there (see preceding verses), but do not seem to have paid the tax. Perhaps they were young enough to be exempt from paying it, and so were still teenagers.

We know that a few of them had established jobs including being a fisherman (Peter, Andrew, James and John) or tax-collector (Matthew). But as Jewish schooling normally finished at age 12, there would have been time for them to learn these trades and still join Jesus at a young age.