Everyone over the age of 55 was once 50. Everyone 50 was once 45, ... Our actions at age 20 were conditioned on our expectations of the future.
By your logic, if I am 49 and robbing a liquor store, I might as well kill any witnesses, and reduce the chance of a conviction, since the penalty for armed robbery and multiple first degree murders is the same: six years.
Its who to lock up. Prison space is expensive and we ought to reserve it for the worst of the worst and those who constitute an ongoing danger to society.