Not too young... the first book mentions millions of people dying in various famines, wars, etc, with nice infographics. That said, my 10yo read some of it this past summer and was fine (I hadn't expected him to read it, he just picked it up because I had it lying around), but iirc he didn't touch the 2nd book in the series, nor did he ask for the second book.
ETA: I just looked it up. When the Ming dynasty fell, 95 million people perished (book 1, page 29). Dozens of millions for most of the other dynasty changes (71% of the population perished when the Han dynasty fell). Also, some of the words would be too hard for young kids (it's written for adults, iirc). So, middle school and up-ish and ymmv-ish?