its the numbers based over humanities experiences with murderers. ^.^
here, i found a lil snippet for you
The age pattern for homicide offenders follows a pattern similar to homicide victims—that is, homicide offending rates decrease with age. According to the UCR data, the average age of homicide offenders fell from 29.6 years in 1980 to 26.4 years in 1994, and then increased to 28.8 years in 2008. From 1980 to 2008, young adults between the ages of eighteen and twenty-four consistently had the highest offending rate of all age groups.
(victims are usually younger as well,.)
I knew about this fact forever now though.