The thirty Aeons are not typified by the fact that Christ was baptized in His thirtieth year: He did not suffer in the twelfth month after His baptism, but was more than fifty years old when He died.
– Irenaeus, Against Heresies 2.22
Irenaeus, the first Christian who names the gospels – the first Christian who says “according to Mark”, “according to John”, etc – says that Jesus was over 50 years old when he was killed.
Some people have a theory that the gospels themselves were written in reaction to the Bar Kochba revolt in 133 CE. Jesus’ “anachronistic” prophecy about the abomination causing desolation standing where it doesn’t belong (the Emperor Hadrian erected a statue of Jupiter on the sacred ground of the demolished 2nd Temple which sparked the Jewish revolt), false Christs (Bar Kochba), the diaspora (people fleeing to the hills)… all of that is due to the author of the gospel(s) writing a contemporaneous reaction to Bar Kochba.
Why would Jesus say “let the reader understand”? (Mark 13:14, Matthew 24:15)
That’s him breaking the fourth wall. Quite possibly, since Jesus only does this in Mark and Matthew but not in Luke or John, that Mark/Matt were written first and Luke and John were written “in reaction” to Mark/Matt. Luke supposedly wrote to explain the delayed parousia, which is why in that gospel Jesus’ interpretation of the “mini apocalypse” is different than Mark.
The first Christian to cite a narrative gospel was Marcion in 140 CE. There doesn’t seem to be any direct evidence of narrative gospels prior to Marcion. Papias cites an unordered sayings written by Mark, a disciple and interpreter of Peter. Ignatius writes a couple of things that seem to be from Matthew, but he never quotes Matthew directly, and they’re not “exactly” how they are in Matthew – meaning that he might just be reciting traditions.
So the gospels being written later than what the vast majority of scholars say they were written (70 CE for Mark, the first gospel written) makes sense of Irenaeus’ assertion of a 50 year old Jesus. Hell, even John could have been written first! But still… this needs to explain the existence of Christianity itself without any sort of narrative gospels for almost 100 years. It might be that Mark, therefore, has Jesus retroactively preaching “a generation” (which is traditionally 40 years) prior to the destruction of the Temple… the 30s CE. Was there a historical Jesus? I don’t know either way. Christianity could be explained by Philo’s and Paul’s teachings, which were also happening in the 30s or 40s CE. Both of which taught a mediating, heavenly “Christ” functioning between humans and YHWH.
Anyway, if current Christianity takes Irenaeus’ authority on the names of the gospels and how many gospels should be in the canon, why don’t they take his authority on how old Jesus was when he was killed? After Marcion, the next two Christians (that I know of) who cite narrative gospels are Justin Martyr and his student Tatian. But they cite unnamed “Memoirs of the Apostles” and “Diatessaron”, respectively… which seem to be harmonizations of our current gospels. Thus they wouldn’t have the contradictory birth narratives of Jesus found in Luke and Matthew. Luke has Jesus being born during Quirinus’ census (6 CE) and Matthew has Jesus being born during the reign of Herod the great (who died in 4 BCE). Both of those dates have Jesus being around 30 during the reign of Tiberius.
There might not have been gospels with our current birth narratives prior to Irenaeus, which is why he has a 50 year old Jesus.