So, where does that leave us? Fern is tipping us off to the fact that Langdon isn't your average human man. Our best guess is that he matures at a rapid rate. After all, when Vivien is pregnant with Michael, he's fully formed after six months, so strong that he's siphoning the life from his twin.
Still, there are some strange inconsistencies. The jump in growth between 2012 and 2015 seems to be very slight, while the leap from 2015 to 2017 is much more substantial. And in the four years between 2017 and 2021, when Michael arrives at Outpost 3, the changes are even more remarkable. Is it possible his growth accelerates with every year? At the same time, will it plateau at some point? Will he simply remain a grown man who walks the Earth as Satan's son? Or will he age like the rest of us, or much faster?
These answers are still not available, and there's not much theorizing we can do. The reality is, though, we may never have to wonder; we assume the season will come to some grim conclusion without having to talk about the later phases of Michael's life.