Hi there,
Mozart's D minor is probably his most difficult, in my opinion.
It's quite tricky for those who haven't played a concerto, or chamber music before.
I guess, in terms of playing all the notes, it isn't quite difficult, like a lot of Mozart.
But it is extremely tricky musically, not to mention it would be wise to play a lot of Mozart before this.
You might want to try some other ones, they might be too easy for you but they'll act as building blocks for the future:
Haydn - Keyboard Concerto in D major
Mozart - Concertos in A major K414, K488
Mozart - Concerto in F major K459
I think Grieg and Schumann will be alright for you, they don't seem difficult, I've only studied the Schumann. I'm not saying Mozart's D minor concerto is too difficult for you, it's just that I'd recommend you learn an easier concerto to get used to playing concertos, and then learn it.
Good luck!
I'm 14 too, around the same level I guess, just haven't bothered to get the ATCL because it's a bit difficult to apply over in my country. Actually, I'm learning the third Ballade now