It has been theorized that the first born (newborns?) could have been been killed by the poisoning or contamination of a food product that would only be fed to newborns. It stands to reason that the diet available to the Egyptians was different/better than the diet available to the Hebrews, hence the Hebrews first born did not eat the contaminated food. That's just one theory.
My primary point in this thread is that I believe some version of these events did in fact happen. It may not have happened in the way that it has come down to us through literature, history, or Hollywood, but the outlines of the narrative correspond too well with what we know from modern science. The order in which the plagues occur supports the veracity of the story; what may have seemed impossible or miraculous to previous generations can now be explained as rare but natural phenomena that we now all understand--like the cloud formations above!
But, that's under the assumption that only infants and toddlers were killed. Scripture says the firstborn, which isn't limited to children. Firstborn males (20 or older) were often leaders of their households, when their fathers died or were too old to run the family business or lead their clans.
Remember the scene in
The Ten Commandments, when Pharoah Rameses is planning the strike on the Hebrews and the angel of death shows up. The general says that he's known battle for 30 years but has never known fear until this night. Rameses orders another soldier to muster the troops and prepare to attack Israel.
The mist surrounds this soldier and he drops. With his dying words, he pleads to Pharoah,
"Let the Hebrews go, great one. Or we are all DEAD MEN!" The general holds this soldier as he dies. Rameses asks him,
"Is this your son?"; the general sadly replies,
"....my firstborn!"It is then that Rameses remembers the words of Moses and rushes to his son's room to see his queen at the bedside of their dying boy.