I read a blog relatively recently that concluded with a theory that the Apostles, with the exception of St. Peter, were all adolescents between the ages of 15 and 18 when Christ first called them to follow Him. The author used some rather compelling evidence. For one, the education of Jewish males was complete by the age of 15. By the age of 18 they were expected to have taken a wife. Yet, despite this, the New Testament is pretty mum on the marital lives of the Apostles, save one - St. Peter. We are told by both St. Matthew and St. Luke that Jesus healed Peter’s mother-in-law. So of the Twelve, it is very likely that Peter was the only one above the age of 18.
In fact, this is reinforced by Matthew’s Gospel when Jesus tells Peter to pay the temple tax for the both of them. The temple tax was paid by Jewish males aged 20 and above and Jesus only tells Peter to pay the tax for the two of them. None of the other eleven are mentioned. So it stands to reason that Peter was significantly older than the other Apostles, perhaps already in his mid-20s, when Jesus’s ministry began.