I was thirteen going on fourteen when I first read It. It was my first experience of Stephen King and I was hooked instantly, devouring the book in five long nights (school was not my friend that week). It made me want to be a writer (still working on that) but it resonated with me in hundred other ways as well, and most of that I attribute to my age at first exposure. There are a lot of shocking things in that book, but back then the sex scene wasn't one of them.
I keep coming back to it (or It, I guess), it is my favourite book (not just my favourite Stephen King) and the older I get the more troubling that scene becomes, but that's fine, it's been a long time since I was thirteen. In some respects I actually think that It is a kid's book, just, you know, not one you would necessarily want your own kids reading.