So, likewise, he was an old man for old men … Now, that the first stage of early life embraces thirty years, and that this extends onwards to the fortieth year, every one will admit; but from the fortieth and fiftieth year a man begins to decline towards old age, which our Lord possessed while he still fulfilled the office of a teacher … those who were conversant in Asia with John, the disciple of the Lord [affirming] that John conveyed to them that information. … Some of them [i.e., those who teach this, PS], moreover, saw not only John, but the other apostles also, and heard the very same account from them, and bear testimony as to the [validity of] the statement.
(Irenaeus, Against Heresies
Irenaeus takes the source from the fourth gospel. But just the fourth gospel has trouble with some people identifying dangerously John the Baptist with Jesus.
He himself was not the light; he came only as a witness to the ligh
19 Now this was John’s testimony when the Jewish leaders[a] in Jerusalem sent priests and Levites to ask him who he was. 20 He did not fail to confess, but confessed freely, “I am not the Messiah.”
21 They asked him, “Then who are you? Are you Elijah?”
He said, “I am not.”
“Are you the Prophet?”
He answered, “No.”
22 Finally they said, “Who are you? Give us an answer to take back to those who sent us. What do you say about yourself?”
23 John replied in the words of Isaiah the prophet, “I am the voice of one calling in the wilderness, ‘Make straight the way for the Lord.’”
Hence the probability increases, that both the fourth Gospel and Irenaeus :
- feared the confusion between John and Jesus
- were victims partially themselves of this confusion.
This partial confusion would have provoked the view of a Jesus 50 years old.
Because "John the Baptist" lived under Claudius.
It came to pass, while Fadus was procurator of Judea, that a certain charlatan, whose name was Theudas, persuaded a great part of the people to take their effects with them, and follow him to the river Jordan; for he told them he was a prophet, and that he would, by his own command, divide the river, and afford them an easy passage over it. Many were deluded by his words. However, Fadus did not permit them to make any advantage of his wild attempt, but sent a troop of horsemen out against them. After falling upon them unexpectedly, they slew many of them, and took many of them alive. They also took Theudas alive, cut off his head, and carried it to Jerusalem.
(Josephus, Jewish Antiquities