When I applied to d-school, sa couple of my pre-reqs were 14 years old. I applied to 8 schools. None of the schools considered my application incomplete because I hadn't taken the pre-reqs within a certain timeframe. Now, I may have been fortunate enough to apply to the right schools, but I highly doubt it. I am sure there are some schools out there that require the pre-reqs within a certain time frame, but don't just assume that most schools have this requirement. I would check the schools you desire to apply to and see what they say. I know I didn't have to take a single pre-req and all were 11-14 yrs old.