Oh it's possible to add an intermediate page, but it's not easy in the least. Something that would be easy - add a warning like the text box at the top of Practical tactical. Obviously VBulletin has a feature that lets you add those easily. What you're asking for is a gateway.
Remember, Spark did not create this website - he installed and configured VBulletin, which is just software someone else programmed. All the features are already decided, and adding something like a configurable gateway feature requires a ton of work for a programmer - it's about impossible for the vast majority of forum owners that just install and configure a platform like VBulletin.
The one way to add features is to find a plugin. PLugins are designed by other programmers based on if there is enough need that they can make money on it. Most are very weak - like, there might be a gateway plugin, but it puts up the gateway on every single forum. Also, every plugin is another security issue because it's another thing that hackers can exploit. You have to keep them updated, and you have to hope that the creator will keep updating it indefinitely. Most plugins cost $$$, and the ones that are free usually include some malicious code that allows the creator to get payment out of you one way or another. I'm not an expert with VBulletin, but I would guess that there's a 95% chance that it doesn't have the feature you're asking for built in, and there isn't a reputable plugin available that is powerful enough to configure it to do exactly what you're asking and nothing more.
Now, it is possible to hard code a gateway into the forum with deep edits. The problem there is that even if you're a professional programmer, it's extremely hard to add a feature like that to a platform that has several hundred files already. You have to engineer a way to integrate the feature into the existing framework, then you have to code .PHP files. YOu have to integrate that code with existing .PHP files. You have to write the .HTML files for the pages that will be displayed, and you have to add the the .css files that configure the new attributes. Now, you also have to add the control features to the back end, and add code snippets through many files in the form of if-then statements that say "If the gateway is turned on, redirect through so and so file / if no, then void this operation". Since you're editing core files and files dealing with the backend, you could mess up the forum or even mess up the control panels. There could be code already existing that interferes with the integration of such a feature, and the code you write could possibly interfere with any number of other commands, files, and features. Just for an example of how involved edits like this is, you can pay a programmer $500 to install, configure and add a pre-conceived design to a Wordpress website. If you asked them to Design an entire theme, you would spend $5,000 minimum for a very basic theme.
There might be an easy way to integrate it, but then all a kid has to do is click "Yes I'm 18" and they're in anyway - what exactly is the point in going through all the trouble?? If there are no quick fixes, it's just not going to be feasible to go through all the trouble for a gateway that doesn't really do anything anyway; that you're wanting to give you a false sense of security.
The reason I said don't mention is this: it's not an easy fix like I said, and it's not very effective. Unless there is a turn key solution for cheap, it's not going to be worth the investment in time, energy and money. As far as liability, there really isn't any legal exposure; all Forums include COPPA statements, and the only Internet law with teeth concerning minors is the COPPA, which says that you cant collect personally identifiable information from minors without an express written consent form signed by their parents. Since so much personal information is submitted and shared through forums, most of those scripts will include the COPPA in their programming. By agreeing to the BF TOS, you're asserting that you're >13, and by visiting the site you're surfing on your own free will. As a paid member to have access to the more explicit forums, you have to provide some kind of payment which comes from an adult. There are many layers between real liability and the website. However, just because there is no real legal grounds Doesn't mean there can't be a case. You can file charges on anything, and let the courts make a decision - that's a very nasty, costly and aggravating process. One thing that everyone is doing by whining and making several threads about this is wiping out any deniability that somebody would have - nobody can say "I wasn't aware that this was a problem anymore". So despite the lack of legal liability, it could be in the best interest of certain parties to nip it in the bud to avoid any claims or cases whatsoever. Since the implementation of the gateway is most likely convoluted, costly, and involved (although technically possible), and simply deleting the forums with mature content is so easy, cheap and straightforward, the solution to this problem, if it exists (and if enough members make it a big deal it's eventually going to constitute a problem) would be to take the easy route.
As for porn, if I was 13 in 2014, I would be streaming HD video from a dedicated porn site instead of hoping that one of the perverts on an unrelated forum shared my same tastes in brunettes with a thigh gap, and gambling that they only posted from age verified sources (temporary Internet files are physical files on your computer). I rarely visit the threads in question, so it doesn't matter to me really what happens - but I know a lot of paying members do care a whole lot, so I wouldn't want to do anything to affect it for them unless there was an extremely pressing reason. The reason I don't visit them much? Not much need since my early high school years - oh the irony.
I'm definitely not advocating that people not discuss what they want though. If it's a pressing concern for you have at it, but there are certain truths that aren't going to change no matter how many people support something - like how difficult it is for someone that just installed an application to re-write it (it's like me telling you to add a function to your Internet browser - you're not really a programmer just because you installed Firefox are you?), how available a plugin is (if it doesn't exist, it doesn't exist. If existing plugins aren't powerful enough, there's no easy way around it), or the fact that the solution that the majority of people do not want is by far the easiest, and if everybody pushes for a solution they just might get one. I could be wrong too - there may be integrated features in VBulletin that make those changes a whiz, but I do not recall any configurable gateways that do what you're wanting. It's not a matter of supporting your idea if the solution isn't feasible. It might be a choice between letting everyone access the mature parts of the site, or letting no one.
You folks take care!