AIUI this is actually one of the concerns behind the minimum age recommendation in the literature.
The brain combines multiple sources of information to estimate how far away things in the world are from us. From our visual system these include: correlation between the inputs from the two eyes; shapes and patterns detected within the scene; motion parallax; the distance at which our eyes are focused; and the angles from the point on which we are focusing to our eyes.
A problem with current VR tech is that some of these pieces of information do not match each other when we are wearing the VR headset. Specifically, instead of accurately recreating the light field that our eyes would be exposed to for a given 3D scene, the scene is instead presented in sharp focus a short distance away from each eye. The angle through which our eyes have to rotate towards each other to focus on a single point, the shape the lenses have to take to bring that point into sharp focus, and the relative positions of that point in the two scenes our eyes perceive do not have the same relationship to each other that they would in the real world. Moreover, while wearing the headset, we make deliberate effort to interpret what we see as a deep three dimensional scene, which has the effect of training our brain and visual system to just rely on the stereo vision aspect while ignoring the other conflicting sources of information. Some people are better at this than others, and this leads to some of the variation between how ill people feel after prolonged VR sessions.
We know that the brain is very plastic here - e.g. after just a day or two of wearing mirror glasses that flip what we see upside down, we can reliably learn to compensate for that; people who only have good sight in one eye still have some depth perception; etc. We know it's more plastic in kids, and in young kids is still in the process of learning how to synthesize information from various sources to create reliable depth perception. Last time I worked in this field, I was not aware of any firm results in research quantifying relevant ages or long-term effects. A few minutes of VR is very unlikely to cause any lasting harm. Hours or days will have noticeable effects on adults, never mind children - though I know of no results that show whether any such are anything other than short-term, given that we equally do not know if there is lasting effect, some level of caution with respect to long/regular sessions seems prudent.