The big thing about It Chapter Two is that it’s supposed to take place 27 years after the events of the first IT because Pennywise the Dancing Clown loves to attack the poor town of Derry, Maine every 27 years. Knowing this, we’ve now got adult actors playing the IT Losers Club kids we met in the first movie, now just aged up roughly three decades. As for the kids we met in movie, they’re here, too, and they look absolutely identical to how they were in the first installment — which was filmed three whole years ago in 2016. Considering that this new sequel was not on anyone’s mind back in 2016 how is it possible that a bunch of tweens and teenagers haven’t gone through any sort of puberty and or growth spurts? When was IT Chapter Two actually filmed?
Well, despite appearances, those kids have grown up. In reality, they actually look very different than they did during the filming of the first movie. But the reason you didn't get that feeling while watching Chapter Two is that the kids have now been digitally de-aged to look like they did in the first movie. Oh yes, we’ve entered a world where people are digitally de-aging Finn Wolfhard. Honestly, it was just a matter of time.
Of course, if anyone had known that the first installment of IT would be a such huge success, the Losers Club kids might have filmed their scenes for both movies at the same time, since that would 100 percent maintain that visual continuity. However, that didn’t happen. IT Chapter Two was only green-lit 20 days after IT's 2017 release. That means that the kids returning for the movie were two years whole older when filming on IT Chapter Two commenced in 2018, and we all know how quickly kids grow between the ages of like 12 and 15 (that awkward phase full of growth spurts and changes isn't something any adult manages to forget).
Talking to Total Film, the director of both IT films, Andy Muschietti, explained that the workaround for the passage of time was always part of making the second film, as they knew the kids needed to look a little bit younger, but not by much. “In those two years [between films], the kids grew up quite a bit. Not all of them. Sophia looks exactly the same. Jaeden looks pretty much the same. Finn grew up quite a bit, and he’s a tall guy. But from the beginning, we knew that that would be part of the budget, the visual effects to address that. So we’re going to de-age the kids.”
From the looks of the new movie, it appears part of the de-aging also involved shrinking Wolfhard down a bit too. In case you hadn't noticed, he currently towers over the rest of the kiddos at 5’10”.
So no, your eyes are not deceiving you: The Losers Club kids don't appear to have aged. And piles of money have been spent to ensure that.