In reality, police officers in the United States are the most looked into and evaluated persons around. Both physical, mental, background, personality, family, etc are tested, screened, interviewed, evaluated, compaired. ect....
What other occupations are like this? Not many.
It can take a year to get a LE job in most instances due to the screening process.
***Though, the screening process is not perfect*****
I can speculate on how and why anyone would hire a 20 year old to be an officer. Not enough life experience IMO. There may be instances when hiring guys from the active military, who have been around on their own and in rough situations, yet are young, as officers. I just have yet to know many early 20 something's with the maturity and life experience to be placed in such a variety of situations police officers now days are.
IF I am rambling, sorry. Lack of sleep.
I know the cops in Germany are screened well. The ones I ever saw were atleast 6 feet plus tall, lots of muscles too.
Then again there is no such thing as police brutality in Germany. You do as they say or you get a baton across the head.