You are speaking of the Great Leap Forward hypothesis which, depending on which variation you chose, says that fully modern humans didn’t arise until 100,000 or 50,000 (or somewhere in between those two) years ago.
Language ability has long been a favorite guess at the key difference, but it’s not the only possibility.
It should also be noted that average cranial/brain size for H. sapiens and gone down slightly from neolithic times. Why? dunno. My personal guess might be that our brains became better organized, allowing us to do the same, or even more, with less. This would be a good thing for a number of reasons, from making the birth process slightly less fraught with problems to saving on the energy needed to grow and maintain more brain tissue (which is greedy for both oxygen and calories). But I’m not a scientist, so consider that a just-so story/possibility.
Back to the OP - depending on whether or not the Great Leap Forward thing is a thing or not, your hypothetical kid from 100,000 years in the past might look like a modern human, but something may be off in their brain. They might have problems with language. They might have difficulty with abstract thought. They also might have a better memory than us (or maybe not) or some other advantage which may or may not make up for what we would perceive as deficits.
Something changed about 75,000 to 50,000 years ago in H. sapiens, as that is when we start to see two things: a migration out of Africa (which could be down to environmental/external factors and not us/internal factors) and a sudden growth (as near as we can tell from the archaeological record) in both complexity and diversity of things like burial practices, tool making/use, art (this is when cave painting took off), long range trading routes (continent spanning, in fact), and so on. Was it due to some sort of critical mass of population and accumulated cultural learning? Or did something “click” the in brains of a small population that later when on to conquer the world, perhaps allowing for modern language or some such?
Actually, are diets are LESS diverse than those of hunter-gatherers, even those living in marginal environments today. HG’s in ancient times existed in highly productive environments that are now taken over by agriculture (or cities).
However, the retain-lactase-beyond-weaning trait didn’t become common anywhere until after the domestication of animals. The kid, most likely, would be lactose intolerant once past toddler stage, and if he (or she) had a problem with mac’n’cheese it would probably be due to the dairy component.
Actually… we don’t know that. We do have evidence that at least some pre-agricultural societies gathered and made use of wild grains so at least some people had some gluten in their diet, but on the other hand they probably didn’t consume it in large quantities on a daily basis, which may or may not make a difference.
There’s been a world-wide trend that when you eliminate intestinal parasites from a population their rate of allergies increases. Presumably, if said ancient baby has worms the child will be de-wormed upon arrival in our time. Will that create a problem? Damnifino.
Likewise, a rise in literacy rates is linked to increase myopia in a population. Left in the past, the tyke would presumably not have allergies and not require eyeglasses but raised in the modern world both of those may ensue due to the change of environment.