Most if not all airlines should allow a 14 year old to travel alone however it's entirely up the the individual airline who puts the rules and conditions in place whether to allow a 14 year old to fly on an airplane without a parent or guardian.
The airlines that I know of that will allow a 14 year old to fly without a parent or guardian are American Airlines, Continental, United Airlines, Jet blue, Spirit, Delta and US airways all allow 14 year olds to travel alone on their airlines.