Bokura no yûki - Miman toshi (1997)

Makuhara city has been off-limits since it was reported that there was a local earthquake. Two high school boys, Yamato and Takeru got acquainted and hit it off while they headed to Makuhara from each city where they live with their own purpose. They sneak into Makuhara as they are surprised by heavily guarded around the city. Makuhara has changed into a confused place where all the adults had died mysteriously, and boys and girls under around 20 years old are surviving.