Wikipedia has nothing on the subject that I could find. Google was more helpful, but I can't find the answer to my specific question.
It's quite simple. Is it remotely plausible for someone to be a homicide detective with some experience at age 27? I know the NYPD won't hire anyone under the age of 21 (and training, of course, takes 1.5 years, besides the time spent climbing ranks), but I can't tell if that's universal for all USA police departments.
If it helps, this is a modern day setting; the character graduated high school at age 17, and, if we need to, we can assume she was the best, the brightest, had the best luck, and tons of favoritism from her supervisors.