How old were the disciples of Jesus Christ during their ministry?
No one really knows. However, the disciples who were fishermen like Peter and Andrew his brother, would have to be fit enough to work hard as fishermen. As life expectancy in that country at that time was no more than about 40 years of age (although some did live to a ripe old age, but this was unusual), it is likely that Peter and Andrew would be relatively young men in their late 20s or early 30s (like Jesus himself - around 30-33).
Matthew (also called Levi) being a tax collector, had a sedentary job and could be any age from 20s through to 40s or more.
No indications are give of the other disciples, including Judas Iscariot, but it is likely they would fit into the same range - from 25 - 40.
However, the disciples James and his brother John were known as 'Sons of Thunder' and their father, Zebedee himself was a fisherman, owning a boat. Therefore, by the same logic as Peter and Andrew, Zebedee would have to be in his 30s at the oldest, meaning that James and John were little more than teenagers. This fits well with the story of their mother asking Jesus about who should sit at his right hand in his kingdom (Matthew 20:20) - the first recorded incident in the New Testament of a 'pushy parent' trying to get the best for her two boys! It also agrees with their title 'sons of thunder' - as two beligerent teenage boys. Finally, the evidence that John was but a teenager when he was a discple of Jesus allows for the fact that he was able to write his gospel much later in the first Century - around 80-100AD, when he was a very old man.