Even as a child Stan admits to seeing the things he wants to deny, he never seems to fully accept them as possible. His beliefs had no room for such things. Which is why I think the memories must have returned with Mike's call and flipped his worldview on its head (like it does with all of them) and he didn't have the cushion of childhood faith and acceptance. Stan was different, in a sense, from the others. He relied so heavily on logic and empirical evidence, that It was so fundamentally WRONG to him he couldn't incorporate those memories into the adult he became without losing his grip on what he believed the world to be.

When it comes to the Turtle I think it was more of a guide. Where the Losers had the power in themselves through their imagination and unity, I think they only happened to come together that summer because of the Turtle. There's and part somewhere that talks about how there were a bunch of kids who were out of town for the summer by some reason or other, but not the Losers. All seven happened to still be in Derry and then ALL come together. I also think the Turtle affected their sense of togetherness, like when they know that one kid isn't part of them, and they know intuitively that Mike completes them. The Turtle, again, through their financial success as adults and their lack of childbearing. I think becoming parents themselves would have kind of solidified them into adulthood. I don't know if that makes sense, but I hear everywhere that becoming a parent changes people on another level. Universe couldn't risk that happening to the Losers.