Rather than giving a hard number, I usually respond by describing the book and mentioning anything I think parents would want to know about. The Fault in Our Stars is about teens who meet in a cancer support group and fall in love. For some parents, that information is enough to make a decision. Others are apprehensively okay with that, but are absolutely not okay with the presence of a neither graphic nor gratuitous sex scene (and by "not gratuitous," I mean "between two people in an unusual situation who know that waiting isn't much of an option and want to get what they can out of life").
I don't give a hard number because you know better than I what your kids already know, what they've already seen on TV, what they're ready to handle. That's true of the brief and really really not graphic sexual content, but I think it's even truer of the larger themes of terminal illness. This book is entertaining and even very funny in some places, and much of its value is in the big questions its characters consider about what makes a life meaningful. But it's also a book that might be upsetting to some readers - preteen, teen, or adult. As with any book, we'll share what we know about its content as honestly as we can. But you know your family and your family's experiences better than we do. So do your kids (and I've seen great, honest conversations between parents and kids who decided together to wait on a book).
If you or your kids do decide to read TFIOS, we'll be waiting with plenty of happy, escapist recommendations when you're ready to choose your next book.