While SptCA's advice is certainly erring on the side of caution, there is no actual way to enforce a PHOTO ID requirement for those under 18, since, unless they have travelled internationally, they most likely do not own one. Yes, it is possible to get a state ID issued in many states, but most people do not do this. I have never been asked to produce ID for my kids except when my older daughter was approaching her second birthday and was flying on a 50% off seat. The agent asked to see her birth certificate to prove she was under 2. We didn't have it and they made us pay full fare. AA later refunded it when we faxed in a copy. But that is beside the point. For security they cannot make a minor show ID.