well my "old school" brush was a BAdger 350 back in 1988 or so... it wokred ok.. but after getting my Crescendo by Badger, then my Anthem... I have to say they are MOST definitely improvements.. I remember with the 350 having to adjust the amountof paint outpu and pressure, then paint one color... clean and do it all over again...
now it is just paint clean paint clean.. no adjustments since it is all on the fly now...
Which thinking about your AB question for a 14 yr old.,.. maybe double action is the way to go.. there is no such thing as a "learning airbrush" in my opinion.. as if he gets good with a single action, he'll still have to learn all over again with a double... why not learn once and slowly get better with the same airbrush? it's only a few dollars difference at Dixieart.com