Prodigy composers

I have a homework question for you classical music mavens to do over the weekend. But before I tell you what it is, let me give you a little background.

When I was a senior math major in college, I was busy trying to find other fields to go into, because I knew I didn't have the talent for a career in math. What I used to say was: "When Galois was my age, he was dead." Galois was a young French mathematician in the early 19th century, who "died from wounds suffered in a duel under murky circumstances at the age of twenty." Before he went to that fateful duel, however, he wrote down some of his mathematical ideas, which later became highly significant in advanced algebra.

Like many mathematicians, classical composers often were prodigies. If you haven't heard of a fellow named Mozart, perhaps you should scroll down to the next post.

Mozart, of course, was not the only prodigy composer. I've been listening to Felix Mendelssohn's Octet for Strings lately, which he wrote at age 16. This is a stunningly brilliant work, and although I really am not the greatest Mendelssohn fan in the world, it's almost impossible to offer too much praise for the Octet.

All of this led me to say that if Mendelssohn had died right after writing the Octet, that work alone would have demanded that history treat him as a great composer.

So, finally, here is my question:

Can you name other works by other composers, written at the age of 20 or younger (easier version: 30 or younger), that alone would have made them a great composer?

I'm not going to grade your answers. I don't have any answers, myself. I was just wondering what you can come up with.