The second movement of Mozart’s String Quartet No. 4, K. 157, part of the so-called “Milanese” quartets, composed in Milan when he was 16, or perhaps 17. I think it’s pretty profound for one so young. The Amadeus Quartet performs; I’m not wild about the performance (too soupy) but it gives you a good idea of the piece. Incidentally, I ran across this on YouTube when the video had a total of 1 view. The post says it was “Auto-generated by YouTube.”
The “soupiness” of this rendition, magnified by that wildly vibrating sound, is definitely palpable. Compared to the famous String Octet by 16-year-old Mendelssohn, this piece does not seem particularly astonishing; but compared to just about everything else created by teenagers, it certainly is highly impressive.
Well, nothing, or very little, compares to the Octet. My son is playing K. 157 with his string quartet, and this movement really stands out among the three.