To establish some context, I'll start by saying that when I was growing up, my parents told me that none of the books in the house were off-limits to me, and if I had any questions about anything I read I was welcome to bring it up with them. I think I was about 11 or 12 at the time. Of course, being the upright, church-going people that they are, my parents didn't have any pornography or anything like that in the house, but they did buy a regular supply of the current best-sellers, with all that entails. The first book I read with my newly-granted freedom (or maybe this was the book that inspired that conversation) was The Poseidon Adventure. The original movie had come out not long before, and the theme song from the movie was a big hit on Top 40 radio, so I was curious. Anyway, I grew up with the idea that, with proper parental involvement, teens should have few if any restrictions placed on what they read.
Of course, now, erotica and books with very explicit sex scenes are a lot more mainstream than they used to be, and I wouldn't be wild about the idea of my own teens reading those. So I'll agree that parents, and anyone who doesn't care to read explicit material, need to exercise more caution now than maybe they used to.
Sex isn't the only concern in deciding the appropriateness of reading material. Graphic violence is something that many parents and readers are concerned about; along with, at least for me, the cause served by the violence. I'm less bothered by reading about a villain being graphically and colorfully disposed of than I would be if the same methods were used against a hero, innocent bystander, child, or animal. Strong language bothers some people. Readers and parents might also object to what they consider sacrilegious content. A handful of f-bombs in a book doesn't bother me, but I've been known to put a book down because of light or disrespectful treatment of matters that are sacred to me. Some people might not like to see people of certain genders or races portrayed in certain ways. Some people might object to a specific political slant. The exact definition of what's offensive or inappropriate is different for everyone. As another example, I'm presently reading a fantasy novel that I believe is generally considered "clean," although it contains at least one fairly intense scene of near-rape, and I'm left wondering why near-rape is considered less objectionable than consummated lovemaking, just because the act isn't completed.
Anyway, the list goes on and on, and the consensus among writers is that you can't please everyone, you're bound to end up offending someone, and the best and only thing a writer can do is to write as honestly as he/she can.
So, to the point. How would I rate my books as far as age-appropriateness and offensive content?
My books feature adult characters, with adult lives and concerns, and contain "mature themes and situations," including sex and relationships, earning a living, death, war, sacrifice, and the struggle against evil forces that disrupt their world. A principle I try to follow in my writing is that everything in the story is there because it's needed. (I'm not perfect at this, but it's what I aspire to.) The corollary to that is, if something needs to be in the story, I put it in. If some important story or character development requires a sex scene, I put in a sex scene, though I leave out any description that isn't important to the point I'm trying to make. However, if some detail of the act is important, I'll include that - but still in the least graphic manner that still gets the job done. My aim is never to titillate the reader - I don't want the reader to be pulled out of the story by thinking about their own reaction to what I've written, I want them to be engrossed in what's happening with the characters (this applies to everything I write, not just sex scenes). On the other hand, if the important character and story business take place before the sex scene, I'll draw the curtain. In Urdaisunia, we don't need to know what Prince Eruz does with his three concubines (no, seriously, we don't); what we need to know is the desperate state of mind he's in that drives him to seek comfort that way.
Same thing with violence. Prince Eruz has to execute some people. The important thing is what's going on inside Eruz's head as he is forced to carry out these executions, not graphic descriptions of the actual deaths, so that's where the focus is in the writing. In Lost Book of Anggird, a couple of reprehensible people are killed in a particularly grisly way as punishment for their misdeeds; the act of carrying out the killings is cathartic to the person who does it, as well as demonstrating the extreme state of mind that character is in, so I focus on that in the writing and not on the (literally) gory details.
Language: I try to use restraint in the use of strong or offensive language, because it loses its impact if it's overused. (A personal line for me is not to use religious oaths that apply in our world; on the other hand, characters in an invented world using oaths that reference invented gods aren't a problem for me.) Again, if it's necessary to communicate what the character is experiencing, or if the use of strong language is appropriate to how the character would speak in a given situation, I'll put in just what's necessary. There's a bit of language in Urdaisunia that's rougher than I would normally use, but it's coming from a thoroughly bad person who is purposely being as insulting and offensive as possible. Anything milder in that situation would have sounded silly.
And so on. In general, I try to put in what the story requires to be told honestly and completely, without going overboard, and certainly without any intent to purposely shock, offend, or titillate.
If you want something more concrete, here is where I rate my books on a couple of different scales:
On the All-Romance Ebooks Heat Rating scale, from 1 - 5 flames, I rate my books a 2: "some [consummated] love scenes. These will be more sensual than graphic and will mostly rely on euphemism." Some parts might edge up just a bit to a low 3, with slightly more graphic description. My short stories run from 0 - 1 flame.
Alternatively, here's a rating scale I devised for fanfiction (another post for another time):
G: Nothing offensive, possibly some slight angst
PG: Occasional mild language, sexual references, mild violence, angsty
PG-13: More frequent language, strongly implied or "on-camera" (non-explicit) sex, more violence, intense angst
R: Strong language, more descriptive (but still non-explicit) sex, semi-graphic violence
M: extreme foul language, explicit sexual description, graphic violence.
On this scale, my books run PG-13 - R. (Short stories G - PG-13).
As far as age recommendations go, my books are definitely not for children. Also not for young teens. As far as older teens go, my books don't fit into the currently-popular Young Adult category, mainly because they have adult rather than teenage main characters and address adult concerns rather than the typical coming-of-age themes usually addressed in YA books, and also may have somewhat more graphic content (though my understanding is that there are plenty of YA books with intense and disturbing content, that address serious issues of sex, drugs, abuse, and so on). Based on my own experience at that age - there wasn't a YA category back then, or if there was it wasn't nearly as prevalent as it is now, and when you were in high school you went from reading middle grade/young teen books to adult books - I think my books are suitable for ages 16 and up. I wouldn't have a problem with my 17-year-old reading them (aside from the embarrassment factor of kid reading love scenes written by his mom!) I mean, really, I'm not exactly writing 50 Shades here.
But, ultimately, the appropriateness of my books is not a decision I'm qualified to make for other parents - or any other reader, period. I hope the information in this post will give readers and parents the information they need to make the right decision for themselves.