Why is it that teenagers were immune to the virus before the virus came into existance?

In the Maze Runner, we know the back story of it is that there are hundreds of immunes.

Thomas is one of them, there are many in the maze trials, but there are also non-immune Gladers such as Newt. What I do understand is that the Flare Virus was engineered by humans to in fact kill many people to ease the stress of the Earth as it was holding more than it could supply. That was the point of the human-made virus.

Why is it that if this virus was engineered by humans...why are teenagers, adults that have lived before the sun flares hit the Earth and babies naturally immune to this virus?