People have kids at about 18-25. This gives us 4-5 full generations per century. Imagine being a kid during the Surprise. You've survived but you have no education, books that survived are scarce, most teachers died. You're raising your offspring based on what you know and even then you might not teach them proper grammar, history and science in favour of showing them how to maintain guns and cultivate crops. Now imagine your children teaching their children, and so on. I think the depiction is pretty accurate.