Why 15? And how could they have a body of work other than taking the SAT ACT etc?
But I recommend 15 because (and I have said this three times already on this thread, but I'm guessing you don't read what I actually write) people paying for college classes don't want to be in a classroom with too many 13-14 year olds. It changes the tenor, it changes what the teachers can teach... it would lead to problems.