The Circle

After MK-175 wiped out 99% of the Earth's adult population, 5 adults remained. They formed The Circle with Joanna Kirk as their leader. The Circle became our government. They decided that teenagers would now come to be the adults of our time, they were given a choice, banishment
or working life.
MK-175 kills everyone at age 21, all except The Circle who are somehow immune.

On judgement day Kira was given the choice - join the sentries or face banishment. She takes the obvious choice and joins the sentries. Here she meets instructor Isaac, the best sentinel alive. She fights to prove she's worth it, while on her journey she discovers more than she bargained for and finds the truth behind MK-175 and The Circle.