I would say that it heavily depends on the child - they're people too, and they will develop differently and have different tolerances for what they can and cannot watch and still be able to separate fact from fiction. In other words, some kids even at 11-13 have a hard time watching "scary" movies like Jurassic Park or Daybreakers, but other kids at the same age have no problem with legitimate horror flicks like Event Horizon or Alien. Upbringing has a role on that, but it does also depend on the person (I know adults that can't actually make it through movies like Alien, it's just part of who they are). The ones that can separate fact from fiction are usually no worse off seeing something like Rotten.com than they are watching Doomsday. The easiest guideline for "when they're ready" in this respect is "can they watch it without nightmares?" - and some kids will probably never get to that point (again, example above). Those kids probably won't go looking out for shock images or other gnarly things though, and the best you can probably do for them is establish and open-door policy to talk about scary things they've seen (e.g. someone may send them a shock image to mess with them, or they might watch a scary movie at a party, etc - just because they know it scares them doesn't mean they have good judgment; they *are* tweens/teens after all).
Porn is in a similar vein - are they able to understand and process that it's make-believe? If they're able to say "this is made up" or "this is fiction" - it goes a lot longer ways to them being able to endure, ignore, shrug off, etc the various stuff they'll find online. And again, the more sensitive souls out there probably aren't going to be looking for skin mags, let alone hardcore stuff. Also, how they were raised and taught about sex is another big factor - is it a forbidden fruit that they're unallowed to discuss, ask about, etc and that gets passed over with innuendo and giggles when its brought up? Or is it seriously discussed as just yet another activity that consenting adults can partake in, and explained as something that much like driving a car or drinking alcohol should be treated with respect and a safety-minded attitude? That makes a BIG difference. I'm not trying to argue the whole "abstinence vs protection" debate that gets brought into schools far too often - what I am saying is, even if your family rule is "no sex outside of marriage" they should still know what sex is, what function it serves, etc - just like for example I'm guessing you probably have a rule along the lines of "no going potty outside of the bathroom" but they still know about bodily functions and what function they serve. Where I'm going here is: kids (especially teens) are very curious about things like sex, drugs, etc; if they can't get the information they want from trusted authorities (mom, dad, preacher, teacher, etc - remember most of the non-mom-and-dad types can't even discuss a lot of this stuff anymore, at least in the US) they'll go looking for it on their own. But you can't expect them to have the research acumen or the discrimination capabilities of a collegiate - so who knows what they'll find, what they'll learn, etc. And that's where you get them trying to learn about the facts of life from 4chan or Erowoid or whatever else. Putting in parental controls on your PC or router just creates a challenge for them - so now it's not just about "what's beer like?" but it comes "why is this knowledge forbidden? what are they trying to hide from me???" Better to head that off at the source and just answer the question than go down the path of playing warden and thought-police.
Bottom line is: "when they're ready" is a good answer. Trying to shield them away from the world and everything potentially unpleasant within it just doesn't work in my experience - the kids I grew up with that were sheltered and hidden away from the world and then suddenly just handed "you're a man now" at 16 or 18 usually are the ones that have the hardest time with alcohol, pornography, etc - just like the kids who were never taught about money management, saving, etc are the ones you see drowning in credit card debt in college. Don't send them out unprepared or ill-equipped and expect them to magically make the best choices ("For though we walk in the flesh, we do not war after the flesh").
This discussion also reminds me of a comic I've seen, I don't think I have the image, but it goes something like:
"Saying I can't teach you about safe sex is like saying I can't teach you about seatbelts and airbags because they'll encourage you to have a car accident." Better to have it and not need it, than to need it and not have it; or as the Scouts say: be prepared.
Of course - everything above is "in my experience" and/or "in my opinion" - your mileage and kids will vary, and at the end of the day you're the parent and have to make the best possible choice you can make with the information and tools you have available.
Another issue that I'm surprised hasn't come up: cyber-bullying. I'd say this is a much more prevalent and insidious problem than sexting or Internet pornography; teaching kids that they don't have to be a victim and can just "walk away" or "log off" is a hard-won trait, but very worth it in my view.
