I've never had this happen with a book, and as far as I know my library doesn't have a policy about restricting material based on age. That said I can remember two times were I got a patron to reconsider checking out a DVD: The first time a mother and her young son (maybe 5 years old?) were checking out some items and among them there was a DVD for the anime Rahxephon (a mecha anime that's basically a clone of Neon Genesis Evangelion). The kid probably thought it was some sort of Transformers cartoon. I pointed out to the mother that it wasn't really age appropriate and the kid would most likely be bored. The second time was actually with an adult. The patron was going to check out our copy of Get Smart: The Complete Series. I pointed out to the patron that this wasn't the series from the 1960s but the reboot from the 1990s with Andy Dick (although Don Adams and Barbara Feldon were prominently displayed on the cover while Dick was tucked away in the upper corner). The patron then declined to check the item out.
