This is not just an American problem because the academic standard is lowering in many countries. This might be attributed to more attention to social media, computer games, and other social activities by the youths. Most examinations like the SAT and ACT examinations allow the use of calculators because of the poor performance of students. The military is supposed to be made up of the best students with the ability to solve diverse mathematical problems without aid because they work in very uncomfortable conditions. They are expected to possess fast problem-solving skills which is why they need to be tested without a mathematical aid. If the shortage of military personnel persists we might be forced to remove any form of intellectual examination.
Computer games have been there for ages already so that's hardly the reason. I don't think there are studies supporting that theory and surely there already would be. I personally can't see them affecting to scores, in fact quite the opposite. People from my generation who played ended up coding and working in IT and some of them in field of mathematics and have PHDs.
One person that i know wouldn't stop playing if it would be up to him, and he is genius when it comes to mathematics. He doesn't use social media though.