Question:

Do you know where should I look for information on how old were the apostles when they joined Jesús?

Answer;

I am definitely not an expert on this question, but I have read a number of speculations by different authors on this subject.  Bottom line, the Bible provides no direct evidence for the age of the apostles.  Scholars have speculated that all of them were less than 30 years old simply because they took the role of student to the "Rabbi" Jesus and this was traditional.  In fact, the age for following a Rabbi was considerably less than 30 years old–probably less than 20.  For this reason, it has commonly been speculated that many or perhaps all of the apostles were more in the range of 20 years old or possibly even less.  Others have speculated that Matthew was a bit older than the others because he had a fairly important position as a tax collector.

In the end, this is speculation, and we simply cannot know how old the apostles were.  We know that John lived until about AD 96 or 97, which is about 66 years after Jesus died.  This does seem to imply that John was almost certainly less than thirty when he began to follow Jesus.

If I had to guess, I believe that most or all of the apostles were less than 25 years or so at the time they began to follow Jesus, but I cannot prove this, nor can anyone else as far as I know.

John Oakes