I know exactly what a "master" is. I've had master classes with Dmitri Paperno, Karen Shaw, Sergei Polusmiak, and a couple not-as-big names, but still older and with quite a "past" and with lots of experience to impart also.
Now I see what you're getting at. You're talking about the term "master" being applied to someone so young.
Well, compared to the artists you just named (and the ones I just named also), I agree there! But that's just the term people have stuck with, so oh well...
And just to let you know, I did learn enough to really improve how I play this movement, especially the slow parts. And who's to say I may not play this in a master class with a "master" in the future?