When did Northern India and N. China begin to dominate in global demographics?

Adam Smith in Wealth of Nations, talks about the development of inland navigation systems along great rivers as one of the major causes of economic and demographic growth in ancient Egypt, China and India.

Of all the countries on the coast of the Mediterranean sea, Egypt seems to have been the first in which either agriculture or manufactures were cultivated and improved to any considerable degree. Upper Egypt extends itself nowhere above a few miles from the Nile; and in Lower Egypt, that great river breaks itself into many different canals, which, with the assistance of a little art, seem to have afforded a communication by water-carriage, not only between all the great towns, but between all the considerable villages, and even to many farm-houses in the country, nearly in the same manner as the Rhine and the Maese do in Holland at present. The extent and easiness of this inland navigation was probably one of the principal causes of the early improvement of Egypt.

The improvements in agriculture and manufactures seem likewise to have been of very great antiquity in the provinces of Bengal, in the East Indies, and in some of the eastern provinces of China. In Bengal, the Ganges, and several other great rivers, form a great number of navigable canals, in the same manner as the Nile does in Egypt. In the eastern provinces of China, too, several great rivers form, by their different branches, a multitude of canals, and, by communicating with one another, afford an inland navigation much more extensive than that either of the Nile or the Ganges, or, perhaps, than both of them put together. It is remarkable, that neither the ancient Egyptians, nor the Indians, nor the Chinese, encouraged foreign commerce, but seem all to have derived their great opulence from this inland navigation.

He then goes on to compare them to Africa, Russia and parts of Europe and attributes the lack of development in these regions to the absence of inland navigation systems.

All the inland parts of Africa, and all that part of Asia which lies any considerable way north of the Euxine [Black] and Caspian seas, the ancient Scythia [North of the Black Sea, part of present-day Southern Ukraine], the modern Tartary [modern Kazhakstan] and Siberia [all of Russia] seem in all ages of the world to have been in the same barbarous and uncivilised state in which we find them at present. The Sea of Tartary is the frozen ocean which admits of no navigation, and though some of the greatest rivers in the world run through that country, they are at too great a distance from one another to carry commerce and communication through the greater part of it. There are in Africa none of those great inlets, such as the Baltic and Adriatic seas in Europe, the Mediterranean and Euxine seas in both Europe and Asia, and the gulfs of Arabia, Persia, India, Bengal, and Siam, in Asia, to carry maritime commerce into the interior parts of that great continent: and the great rivers of Africa are at too great a distance from one another to give occasion to any considerable inland navigation.

The navigation of the Danube is of very little use to the different states of Bavaria, Austria and Hungary, in comparison of what it would be if any of them possessed the whole of its course till it falls into the Black Sea.

Using the delta areas as proxies for the entire river valley systems:

The area of the Ganges delta (about 59,000 km²) is nearly 4 times as large as Mesopotamia (about 15,000 km²), and 3 times as large as the Nile river delta (about 19,000 km²). Additionally:

• Once wet-rice cultivation with paddies is developed, rice provides nearly twice the calories per acre as wheat and other Mediterranean grains; and

• Two crops per year are more common in Indian latitudes than Mediterranean latitudes, and much more common that North and Central European latitudes.

It is clear that, for comparable technology, the supportable population in the Ganges area is:

• 8 to 16 times that of Mesopotamia
• 6 to 12 times that of Egypt
• etc

Clearly the Ganges river system is going to dominate human population statistics until other things are no longer equal.

So as you are asking when the domination began, I would say that occurswhen wet-rice cultivation (developed in China ~ 6,000 BP) reaches Northern India about 4,000 BP.