The American Sailing Assoc is hardly a 'spokesman' for the US sailing community (being a for-profit entity involved in sailing intruction), much less capable of 'setting an age limit'. The age limit you speak of is enforced by the FAA, a government agency.
In any event, Laura Dekker is Dutch. She plans on stopping numerous times, which leads me to surmise that she may well be planning to sail to the north of Australia. She's taking the 'easy' way around, even in the event that she chooses to round Agulhas.
A significantly less risky RTW than what Abby Sunderland attempted and Jess Watson completed. And she has far more sailing experience than either.
So now a 15 year old will try. Do you have a cutoff age when you think the risk is too great?