Any Oneworld FF programmes open to under-18s?
AA is a good choice so long as you don't expect them to earn status. AA has a minimum flight/mile quota on their metal, meaning you can't fly just CX/JL/QF and earn status. And given the future changes when the AA and US programs merge, who knows what you'll be getting. So unless you're needing a plan for pending travel, waiting a couple months might be helpful. But you could always change.