Is such a thing even possible? Movie classification is tied to age, and the assumption that while a six-year-old probably shouldn't be watching the Texas Chainsaw Massacre, a 36-year-old has the emotional and mental maturity to discern fact from fiction and red corn syrup from real blood. But the motion sickness that can come from too much time spent in virtual reality worlds isn't locked by age. In the same way a 10-year-old can laugh their way through a rollercoaster ride that would have me screaming, crying, and praying to any deity that would listen, different people (and different stomachs) have very different reactions to VR headsets.

10-year-olds can handle rollercoasters that would make me cry

And those differences can't be cordoned off by age restrictions alone. Young children are still developing their nervous systems, but going fast is not a uniquely adult experience. Nor is completing a 360-degree spin, or falling down a hole, or understanding that despite the visual input from your eyes, your body is not actually moving. If we want to classify VR experiences by sensation we need some other kind of categories, just as movies and games make it clear when they feature fantasy violence, sex, or "fear."

Oculus has already made some strides in this department. The company, which shipped its first consumer Rift headset this week, categorizes games by comfort level, from "comfortable," through "moderate," to "intense." Both EVE: Valkyrie and Elite: Dangerous have been given "intense" ratings, a decision that seems to make sense — both games put you in the pilot seat of a spaceship and ask you to navigate 3D space, survive dogfights, and track enemy craft, all at speed.

But some early adopters have criticized the current classification system, on the grounds that some of the titles marked as intense are actually less likely to cause motion sickness than other games available for the headset. PC Invasion's Paul Younger says that while he's fine in the virtual reality of both EVE and Elite, the slow-paced exploration game Pollen turned his stomach. In my case, I had no problems bouncing at ridiculous speeds across a neon mountain range in VR music visualizer Frequency Domain, but felt claustrophobic and bug-eyed when asked to walk slowly around a virtual Japanese school.

Researchers aren't entirely sure yet why some VR games trigger such physical reactions in their players, but the inherent sensory conflict involved in entering a virtual world — when your eyes and ears are incorrectly informing your brain that you're moving — is key. So, too, is refresh rate, the measure of how many frames per second a headset is capable of displaying. If this drops below 60 per second, it can start to affect the human brain, making us feel dizzy and disoriented. Studies have also pointed to a person's posture, ethnicity, and field dependence or independence — how much of the background they absorb while looking at an object — as changing susceptibility to sickness, but much of this research was conducted before this current wave of headsets arrived on the scene.

A day in a virtual world is physically taxing

Newer hardware like the Oculus Rift, HTC's Vive, Sony's PlayStation VR, and Samsung's Gear VR, have been developed to cut down on VR sickness, but as some attendees at the recent GDC found out, a day in a virtual world can still be physically taxing. That may be a problem we have to contend with, with certain games restricting the length of time they can be played in one sitting, but others say humans will adapt.

Longtime users, those who picked up the development versions of Oculus' Rift headset when it arrived on Kickstarter, often say it's simply a matter of "getting your VR legs," and training your brain to set out on long VR voyages. In the case of the HTC Vive, Valve boss Gabe Newell says the problem of VR sickness is non-existent, as the hardware's inbuilt tracking system can replicate real-world movements in virtual space, cutting down on the disconnect between brain and body.

As the Rift's competitors hit the market later this year, and VR reaches the mainstream, we'll find out whether Newell is right, or whether our VR games need to come with their own sensation classifications. If it's the latter, we can always just pinch them from movies: PG stands for "puke gently," R-rated indicates imminent retching, and NC-17 indicates you've got about 17 seconds before you're rendered Not Conscious.