You do not see a lot of kids going sub 18 in boot camp. When I went through only 1 kid in our platoon (about 40 guys) went sub 18. He didn't have a running background either. He was kind of a big guy; just athletic as hell. He probably would have been an awesome decathlete.
During boot camp, you do not do a lot of training for the run. Most runs are done as platoon runs which are slow as hell so it does very little to develop speed. Every now and then you are going to get the HS kids that was fast or maybe dropped out of college and enlisted. I'm sure they have a few 16:xx type guys come through every now and then but it is not common.