Why Age 15?
My homeschoolers ( who are NOT unusual homeschoolers) started college at the age of 13, 12, and 13. Two finished B.S. degrees in mathematics at the age of 18.
And...My kids are NORMALLY bright kids who aren't any smarter than any of the kids of any other Freeper.
Even the ones who DO want to go to college... I think you don't have the experience of being in a room full of kids that age. Even bright ones are still often emotionally chaotic. Adults paying for college are not going to want too many little kids in there giggling and yelling out cheeky remarks at random intervals. It's tiresome. I see it even in high school classrooms with a mixture of kids from 9th to 12th grade.... those 9th graders, no matter how intelligent they may be, can be real little jerks sometimes. The 12th graders look at them like a Doberman looks at a yappy chihuahua just before it attacks. Now imagine a Doberman who is paying his own way.