Aging: 80-sai Ijou no Wakamono ga Kurasu Shima

The Japanese government has developed a "rejuvenation drug" that will allow people to return their body state to when they were 20 years old. To test the drug, they have gathered test subjects (aged 80+) on an "island."

Takiro Yabusaki, one of the test subjects, has served a prison sentence of fifty years after being falsely convicted of murdering his wife and child. Now he is determined to track down the real murderer who is on the "island." In order to take revenge, he will infiltrate the "island" in the body of a 20 year old.......

(Source: Comic Action, translated)