Well...how long after the end of the world would it take for people to either become true survivors, or die? I did a little reading on surviving, and people that get caught in such situations and live turn into rather amazingly competent humans. That's because, inside, we are all quite competent at surviving. Our easy-to-digest technical society has made the world safe for laziness and self-indulgence. All of us have relaxed to a level that our ancestors would have sneered at, I think.
Young people are more adaptable, more intelligent, and more durable than we give them credit for. Once upon a time, they survived daily in a world that would curl our hair with fright, these days.
So I don't think it that hard to imagine a group like this that has become efficient at the art of staying alive.