I applied to Carnegie Mellon’s School of Design as an early decision applicant last month, and was checking up on their website and stumbled across a section that said the requirements of applying early, one of which was “Not for applicants under the age of 17.” When I applied, it was October 31st, so I was still 16, and I turned 17 on November 9th. Is this a huge deal and will it affect my early application, or does it not matter since as of now I am 17, and will be 17 when I have to make the decision to attend or not if I am accepted?
I would call the Admissions department and ask; that’s the fastest way to put your fears to rest. I think you’re fine. Call and find out for sure. 
Most likely it refers to when you would start college.
Yea they are probably looking at when you start college. Otherwise this excludes quite a bit of people who were born a little late.
I’m guessing that the statement may refer to Early Admission applicants (as in skipping one or more years of high school) as opposed to Early Decision applicants. My niece was accepted to CMU’s School of Design last year (regular decision) and didn’t turn 17 until March, the same week she received her acceptance letter (she started kindergarten a year early).