I used to be involved in diocesan youth ministry and I took some large groups to  World Youth Day events. I know from experience that the Vatican and the U.S. Church have different concepts of what "youth" means. The Vatican has in mind young adults, even into their early 40s. For us, of course, youth means teenagers. If you look at the difference between the Europeans who attend the WYDs and the North Americans, you'll see that dichotomy reflected.

I would bet that the decision to make 16 the low end of the survey range was probably a concession on the Vatican's part. I can't imagine they'd ever agree to take it down to 14.