Have them go through the standard procedure, application, TS interview, and then a one week ride-along. However now, you would let a Sergeant or above do the ride along, and from that point you can have the Sergeant decide if he is mature enough. If he isn't he would get denied, but if he is mature enough he would get Constable. As Kowalski said there are some immature adults, and some very mature teens under the age limit. So they should be judged solely on professionalism, and character, as well as being mature.