In a recent interview with Decider, Cody Fern talked about Langdon's age. Though he didn't necessarily establish why, he did cop to the fact that there are two Michael Langdons wrestling with reality in Apocalypse: one who represents how Michael looks (the teenager), and one who represents his actual age (the child). Fern said Michael is "coming to discover that he has intense feelings about things" in the world, both "as this emotional 15-year-old, but also as this emotional 6-year-old."
In the same interview, Fern offered a few more hints about this duality within the character. "What's really important about Langdon is that Langdon is the Antichrist, you know? He is the version of Christ who comes from Satan," he said. "There are things that he does that are cruel or evil or whatever you want to call them. But at the same time this is something that he has been born into. So there is a sense of, what is he controlling and what is he not controlling?"
"He is not immortal," Fern also said. "He is a human form of Satan himself."