They know who they're hiring. I'm just surprised they said the quiet part out loud: "It's okay if you're dangerously violent, everyone here is, so long as you only do it in private. "
In a jurisdiction where there is a minimum age to be able to perform the duties of a job, be it for good reason or not, the employer is obligated to follow that law until it gets changed.
Since US police training is relatively shit, and there are plenty of jurisdictions where the age limit is 21. I can't recall offhand at the moment if the post covers where the job opening is/was, but that could be looked up.
Six months of training to start as a cop is not impossible. Again, that's by jurisdiction, there's no single limit afaik. With US police training being shit, a six month span of training wouldn't surprise me at all.
If that's the case, and they're being required the be able to legally carry in their jurisdiction, then dropping the minimum age to apply and be hired for training six months makes as much sense as it only being six months of training in the first place.
Frankly though? I remember that age. I remember how damn many of the people my age at the time were complete idiots. I would be perfectly fine with the age limit being higher than 21 to be a cop. There is such a thing as maturity coming with age. It might not be perfect, but you usually weed out the real idiots by the late twenties.
In the US, at least federally, you are protected against age discrimination only if you are over 40. You can discriminate against younger people on the grounds of their age all you want.
Which, incidentally, governments on all levels (federal, state, local) happily do all the time with inconsistent ages of majority depending on the topic at hand:
16: Age to get a driver's license.
18: Age to vote, enter into contracts, buy a rifle or shotgun, legally considered "adulthood," except...
21: Age to drink, smoke, or buy a handgun, and...
25: Age before which no rental car agency will rent a car to you. (Go figure.)