using the topic Demography in Asia from 600 - 1450, what are some changes in this time period? What are some similitaries? What are the reasons of these similarities and changes?
Among the changes you might consider is the Mongol invasions which dominated large areas of Asia for several hundred years. You should also think about the expansion of Buddhism into China and other areas from India, where it became almost non-existent. Also, the rise of Islam in India and the Mughal (Mongol) Empire there. It was an Islamic ruler who erected the Taj Mahal as a tomb for his wife.
Asia as most of the world remained agricultural. However, you should note that the end of the specified time period was the beginning of the period of European expansion which European influence--and interference-in Asia became prevalent.
Between 1000-1500 C.E. the continuing increases in population led to multiple local religious beliefs being replaced with what is called "universalist" religions. An example is Buddhism. During this period the number of belief systems declines as more and more people accepted the universalist systems. Islam and Christianity were two other universal systems to expand.
Again, this is a massive question that, given the size of Asia as a continent, I would query as to whether is actually narrow and focused enough. What kind of information would you get in response to such a question and would it be useful to you in terms of what you are researching? You might find it more useful to specify a particular zone or region within Asia to focus on.
It would help if we could narrow this question down to a focus on a specific place, such as Beijing or Delhi, for which records are far more likely to be available. One such change would almost certainly be a fairly steady increase in population. I've just read a new book by Niall Ferguson titled Civilization: The West and the Rest, in which he argues that Europe lost a huge part of its population from the Black Death; he never mentions similar plagues in Asia. One point that Ferguson makes very strongly is that most of the larger cities were in Asia rather than Europe before 1450, but that in the last 500 years, many more large cities have grown up in the West.
In searching for this topic on Google Books, you may want to look for "population" rather than "demographics." The former word tends to be more helpful than the latter.
Here's a link that may be of interest to you: