In Against All Heresies, Irenaeus claims that Jesus lived to be nearly fifty years old. The basis for this claim is John 8:57: “Then the Jews said to him, ‘You are not yet fifty years old, and have you seen Abraham?'” Irenaeus argues as follows:
But, besides this, those very Jews who then disputed with the Lord Jesus Christ have most clearly indicated the same thing. For when the Lord said to them, “Your father Abraham rejoiced to see My day; and he saw it, and was glad,” they answered Him, “Thou art not yet fifty years old, and hast Thou seen Abraham?” Now, such language is fittingly applied to one who has already passed the age of forty, without having as yet reached his fiftieth year, yet is not far from this latter period. But to one who is only thirty years old it would unquestionably be said, “Thou art not yet forty years old.” For those who wished to convict Him of falsehood would certainly not extend the number of His years far beyond the age which they saw He had attained; but they mentioned a period near His real age, whether they had truly ascertained this out of the entry in the public register, or simply made a conjecture from what they observed that He was above forty years old, and that He certainly was not one of only thirty years of age. For it is altogether unreasonable to suppose that they were mistaken by twenty years, when they wished to prove Him younger than the times of Abraham. For what they saw, that they also expressed; and He whom they beheld was not a mere phantasm, but an actual being of flesh and blood. He did not then want much of being fifty years old; and, in accordance with that fact, they said to Him, “Thou art not yet fifty years old, and hast Thou seen Abraham?”
There’s obviously a lot going on in this passage besides just figuring out his age, but I actually find this argument convincing, taking the verse in isolation, and in fact, it’s interesting to me to think of what it might mean that the Gospel of John is portraying a much older Jesus than we’re accustomed to.
Yet I believe another passage complicates this reading, because we’re informed of something else that’s not yet fifty years old in the Fourth Gospel: namely, the Temple. When Jesus claims that he can rebuild the temple again in three days, the Jews respond, “This temple has been under construction for forty-six years, and will you raise it up in three days?” (John 2:20). The temple has in fact passed Irenaeus’s cut-off point, as far as I can tell — it’s closer to 50 than to 40, making it an appropriate target for such an accusation. The point seems to be that age isn’t the issue, then, when it comes to closeness to Abraham, etc.
Still, I think there’s room to affirm Irenaeus’s interpretation of Jesus’s “literal” age as portrayed by the Gospel of John — in fact, taking that Gospel as the sole evidence, I’d say you don’t have any basis for thinking he’s only around 30.