Most Americans get drivers licenses at 16, so that, IME, is the age where airlines will expect passengers to have ID. If they have any kind of ID from school, a learner's permit, or a passport, I would bring it, just to mitigate any hassle.
If you are travelling with a minor not your own, you should DEFINITELY get a notorized letter of permission from his parents authorizing you to authorize emergency health care, and a copy of his health insurance card. At that age, he could travel alone, so you probably do not need a letter showing his permission to travel with you, unless you are crossing an international border.