That is, would every first-born son have died, no matter how old, or only the younger sons who had not yet received an inheritance and were still under the authority of their parents? Would Pharaoh have died if he was a firstborn?
ESV - 5 And every firstborn in the land of Egypt shall die, from the firstborn of Pharaoh who sits on his throne, even to the firstborn of the slave girl who is behind the handmill, and all the firstborn of the cattle.
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I read in a book by Jerry B Jenkins who co-authored the Left Behind series that the plague of the first born males would have just been males who were the first to be born. For example, if they had a daughter first, the first son would not die. So pharaoh probably had an older sister.