I'd recommend a Paasche H.
It is simple, rugged, inexpensive, easy to use. You can pick up the H Set for usually about 50 bucks. It has everything you need but the compressor.
The H paints lines that are fine enough to be more than adequate for even 1/72 freehand camo. It is single action and therefore is easier to get the hang of using. Cleanup is snap. The parts are a little less delicate than a higher end double action airbrush.
Eventually, your 14yr old might want to go with a more expensive double action unit, or maybe not. There are not a lot of modeling situations that would exceed the H's ultimate capability. A lot of people recommend jumping in immediately with a double action AB, but I find the myriad details of airbrushing to be sufficiently complicated to get consistenly right that I think it makes a whole lot of sense to start out with a simpler unit.
Contrary to what was written in a letter to FSM editors a couple of months back, the real trick to obtaining a good paint job, is not the airbrush, but rather the skill with which it is used.