They started at ages 8 and 6, and made it somewhere into Book 4. At probably 9 and 7, and lost interest, and have no desire to go back. Granted they have rewatched Stranger Things like 5 times, and I couldn't make it through season 4.
Granted I tried the books, hated them, then watched the movies #1 through #3 on TNT before #4 came out, then circled back and read the books, and loved them. Watched all of the movies, but have also never felt the need to re-read the books, or re-watch the movies. I was studying abroad in England when one of the books came out, and we went to visit some friends in Cambridge. Everything in the town shut down, except the Borders, which had a bar that only served Fosters (which I have have never had before or since). We just read random books off the shelf, drank Fosters and waited for the book to be released. By midnight we realized we were at the front of a line that went down the street, and got interviewed by Cambridge local news