From chapter 26 to 34, the world has slowly begun to end. Due to the weakening of Earth's atmosphere, radiation from the sun has begun destroying almost all living organism. Meanwhile, the bond between Seth and Julia is becoming stronger. Seth's sickness from the slowing is starting to show and it becomes deadly. With the world crumbling down, the last of humankind sends their story away on a spaceship hoping that it would be told somewhere else in the universe.
There has been many themes in The Age of Miracles, but the one that stands out to me the most is it's usually the ones that you wait the longest for that don't show. At first Julia notices Seth but she is unsure if he notice her. After the death of Seth's mother, Seth and Julia started to get close. With the sun's radiation, Seth's illness started to get stronger and with the hope of coming back to Julia one day, Seth and his father move to Mexico where the radiation are weaker. After a short email from Seth, Julia sadly state months later "I never heard from Seth Moreno again". So after waiting for Seth during her whole childhood, he finally comes forward to Julia but only to leave a short time after.
In the last quarter of The Age of Miracles, the main focus was on Julia and Seth. Julia motivated Seth's actions and vice versa. Another force that controlled people's actions could be the slowing. The slowing could have affected Joel's mind which caused him to end up in Sylvia's house. Julia tries to explain Joel's actions by stating. "No one would have predicted my father to be the kind of man who would abandon his wife and child.... For reasons we've never fully understood, the slowing - or its effects - altered the brain chemistry of certain people".
The slowing in The Age of Miracles has many mysterious effects. The known effect include the weakening of Earth's atmosphere, extreme weather, etc. One of the potential effects is messing with people's brain. If that is true, then it will explain things like Julia's friendship with Hanna after the slowing. It will also explain things like how Joel was with Sylvia which Julia explains that it was unimaginable before the slowing.