Historians say that Manasseh co-reigned 10 years with his father Hezekiah. Scripture says that Manasseh was 12 years old when he began to reign (1 Kings 21:1). If they're correct, then Manasseh was about 7 years old when Hezekiah received the prophecy concerning his death (2 Kings 20:5-6).
I was surprised when I read in 2 Kings 20:12 that the king of Babylon sent letters and present to Hezekiah. Assyria and Babylon just don't seem like the kingdoms that would do that - especially considering that Sennacherib had just a year or two earlier invaded Judah. So, I looked up Berodach-Baladan, king of Babylon, on Wikipedia. Sure enough, he was one of the kings in Babylon who assumed rule of Babylon separate from Assyria. When I charted out timelines for this period, it seems that Assyria was the dominant power in Mesopotamia and that they either ruled directly over Babylon or else appointed vassal kings there. However, it seems that some tried to assume power as king of Babylon on their own. (I'll have to read more on this...) It wasn't for about another 80 years until Babylon arose as a power and finally conquered Assyria (with help from a few other nations). At any rate, during the time of Hezekiah's illness it seems that Berodach-Baladan had risen to be king of Babylon for a brief time. I wonder if his action was an attempt to gain support from Judah against Assyria? I don't know - I need to look more into the Assyrian/Babylonian histories and primary sources.
Since Samaria fell during the reign of Hezekiah, Manasseh would never have known a Northern Kingdom (2 Kings 18:10).
Which of these three Assyrian kings carried off Manasseh king of Judah: Sennacherib (the one who invaded Judah), Esarhaddon, Ashurbanipal (the one with the library)? I'm not sure that we know.