The very important fact that everyone has overlooked, as far as I can tell from scanning the posts, is WHEN the plague happened that took the lives of the firstborn, be they 90 years old or 9 months old.
The Bible is divided into two very large sections: the Old Testament and the New Testament. They are called that because of the New Covenant with man made in Christ completes the Old Covenant made with Moses.
Okay, so what does that mean for our scenario? It means that the people of that time were living under a very different covenant with God–the Noahic Covenant, not even (yet) the Mosaic Covenant. And before you begin to ask what does it matter, it matters a whole lot.
God’s covenant with Noah lasted until the one with Abramham, in which he established Jacob and his descendants as his own, and then with Moses. These covenants were limited in scope and purpose. They were not meant to give the whole world’s God grace–that was what happened under the redemption of Christ, but to call men to fidelity to the truth of God.
This means that justice was immediate and deadly for those who violated it and their families because the head of a household was responsible for everything that happened to him and his children. If he made bad decisions, everyone in the family suffered because of his mistakes. This was the system under which they lived–a partiarchal one that had very little room for mercy for one’s enemies.
God chose Abraham and his descendants to be his special people, his family, so anyone who opposed them were enemies of God’s family, not merely of individuals. If they decided to oppose or oppress God’s family theirs suffered for it. This was temporal punishment, not eternal, which is a whole nother topic. By the decisions pharoah made for his people, his family, in defying God, he brought down God’s judgment on himself and his whole nation.
When Christ was born it was the beginning of the New Covenant with man–one in which God poured out his mercy and grace on all man because all man became Christ’s family–his brother, sisters, mother, etc., as Jesus said. In that statement he was declaring the New Covenant in himself that now encompasses the whole of humanity, which is why God does not send punishments upon his enemies now, since all men have been reconciled to him in Christ.
I hope that helps.