My young friend in Canada, Alexander Lang, writes to me that he is starting to work on Bach's Italian Concerto. His repertoire up to now, "but not warmed up" (sounds like eggs for breakfast; in my terminology, it's "in the fingers"), is mostly Chopin; Scherzos 2 and 3; Ballade 1 and 4; Etudes 2, 5, op 10; 12, op 12; Andante Spianato and Grande Polonaise; Nocturne op 48 #1. Also Liszt's Wilde Jagd, Hungarian Rhapsody 2--"can barely manage it" (me too); a few Beethoven sonatas; some Debussy.
He has been reading my book on piano technique (Mastering the Chopin Etudes) and "trying your relaxed-hand technique and it makes a huge difference to especially my octave passages--they hardly take effort anymore. I also really like the single-note approach to technical passages, not going for sequences but individual notes and chords. I bet I could straighten out the Rhapsody with a little work."
First of all, let me advise Alex and everyone else fortunate enough to be 15 or 16, that whatever they are able to get "in their fingers" now will stay with them for life. There will be plenty of refinements and modifications, but there'll be plenty of time for it. Whatever one tries to learn later is not learnt in the same kinesthetic way. It has becoming increasingly sad for me that I realize everything I can actually perform was learnt before the age of 20. I have learnt other things since, but they all disappear from my mind and fingers no sooner than I perform them once. Everything learnt in my teens is with me in my blood; I wouldn't bother consulting a score for any of the works I learnt then in order to perform it now, even if I haven't played that particular piece in a number of years--Chopin G minor Ballade, F minor Ballade, B flat minor Scherzo, assorted Etudes, Nocturnes, Polonaises, etc., Beethoven Appassionata, Moonlight, Waldstein, etc, a handful of Liszt pieces (oh, how I wish I had learnt more!), and alas only two or three pieces by Rachmaninoff. (Now there I really feel sorry for myself, but there are just too many notes in Rachmaninoff--even a small Prelude or Etude-Tableau--to assimilate once one is out of their teens.)
Partly it's because there are too many distractions as one gets older. Did I really sit at the piano 10 hours a day throughout my teens and into my twenties? These days I get up after five minutes to write down something I just thought of that I have to do tomorrow--nothing to do with music. But I had a very full schedule when I was young too. I had eight hours of school every day, for God's sake, and school was a seven-mile trip away. There are many more distractions now, more concerns, but that's not the real reason.
Music is just like a language--it is a language. We can learn a foreign language without even thinking about it when we are five. We have to think about it, but we can still learn it fluently if we really want to when we are 15. But 25--faahget it. No point in even trying, as every German, Dutchman, or Chinese will speak English much better than we will ever be able to master a few phrases in German, Dutch, or Chinese. And we can't do it anyway. Something has definitely altered in the way we learn between age 5 and age 25. As a five-year-old we learnt much more by observation (almost entirely), as an adult by analysis (almost entirely). Maybe analysis has its virtues, but for learning a language--a means of communication (and that includes music)--it doesn't hold a candle to observation and imitation. I was trying to imitate Horowitz when I was 15. I can now analyze everything about his playing (and there's plenty to criticize as well as admire), but my current method of learning would not have enabled me to play like him, whereas my former method gave me some ability in that direction.
What's the answer? Be like a child all your life? Not possible to do. The brain just has its own way of developing.
So learn as much as you can by the first method while you can still do it--get it "in the fingers," or the language "on your tongue." There'll be plenty of time to correct the grammar and spelling later on. Seventy or eighty years. And you'll never be able to learn new repertoire so that it sticks, or a new language so that you can really speak it, in all that time.