A carbohydrate based diet is inflammatory and will eventually lead to insulin resistance and type 2 diabetes for anyone. How long that takes to develop depends on a number of biological factors, but it is metabolically damaging to everyone.

If your ancestors have been eating grains for 10k+ years then you are likely more biologically equipped to process them, as opposed to people descended from a hunter/gatherer population. That's why the rate of type 2 diabetes is so much higher today in the native American population for example. But a carbohydrate based diet isn't really good for anybody.