Yea I was reading up on that. Apparently they are focusing on a younger incarnation of It. The entity known as It the clown is millions of years old, not from planet Earth, and not a clown, and not a spider either. The spider is just another one of the shapes it takes. It can take any shape, he could be an adult, a child, a spider, a clown, a dog, whatever he chooses. He often takes the shape of a clown or a spider but that isn't his true form.
I guess what I'm saying is that its alright that Skaarsgard looks young because it doesn't conflict with the story. It can be anything, use telekinesis, manipulate the world around him, use hypnosis, read minds, you name it, he is a god villain. So at least there is that, a younger actor is playing It but it doesn't conflict with his abilities or the story at all. Apparently he arrived on Earth before humans were even here then he hibernated for maybe millions of years until humans appeared then he wakes up every 30 years to feed on mostly children.
The book has a huge amount of detail and I'm considering another marathon read of that 1100 page behemoth before Sept. 8th when the movie releases.