It depends on the specific airline. I flew alone the first time when I was 7 years old. Most airlines have an unaccompanied minor policy, which can differ from airline to airline, but I believe most start at five years old.
well, i found out that... i think that... well, 14 is the earliest. Try yahoo- that's how i got my answer. I would give the link, but it's not working righte now.