Post-apocalyptic Old man

Shoot away the approaching old men!

2xxx A.D. The world was devastated by repeated wars.
The survivors lived huddled together in fortified facilities called jails.
However, is it a tactical weapon running amok, or is it God's judgment on foolish humanity?
Suddenly, the number of old men increases.
The old men attacked the jails, and in just one month, they captured about half of the jails in the world.
Just when the world was about to be filled with old men, a girl stood up.