Definitely it's not the graphics. I get motion sick pretty frequently, and so cannot play most FPS games. Still I loved games like Doom and Doom 2 and Descent so much when I originally got them(when they came out) that I was willing to give it my all and see if I got sick.
I got just as sick years later with Unreal when it came out, with its far better graphics, and Doom 3, with still better. It's an inner ear thing, exactly like getting sea sick(I get sea sick looking at a glass of water). I feel either barfy or like pooping within five to ten minutes of playing Doom 3. I've even used it to make myself poop when feeling constipated -- works every time.
Interestingly, I can get even sicker when not paying close attention to the game. If I look at t.v. for a while as a descent demo is playing in the background, I will get nauseous VERY fast. Turning head on immediately soothes my stomach. Paying very close fixated attention to the screen and my feeling of what is top and bottom seems to work wonders to keep me oriented, thus not triggering my inner ear that weird stuff is going on, thus not making me sick nearly as quickly or as badly.
It's been many years since I first got Descent when I was in my early 20's I guess it was. I'm curious how bad it will be for me now. But it's not entirely a matter of age. I and many of my friends back in the day got sick at FPS games.
Try what I said, though. I managed to get through the Descent games with only rare sickness. (I think flying around using the automaps in anything but wire frame might have made it worse for me.) You might be able to "adapt" to it like I did.