I’ve been listening to 2 this morning, William Baines (23) and Guillaume Lekeu (24), both new to me. Beautiful piano works. More suggestions like these are welcome.
The other day was the 100th anniversary of Lili Boulanger’s death at age 24, but you know about her already.
You probably mean “composers who died in their 20s”
Because I don’t think Mozart, Schubert, Chopin etc. were that particularly old when they died. 
Check out the Piano Sonata of Julius REUBKE (dedicated to his Master Liszt)
en.wikipedia.org/wiki/Julius_Reubke
en.wikipedia.org/wiki/Piano_Sonata_(Reubke
But I should listen to her again! Thanks. The Baines piano pieces sound particularly good IMO. Another one:
Yes. Thanks for Reubke, never heard of him. Listening to the sonata now, which suggests this for later:
Juan Crisóstomo Arriaga
I think it’s an astonishing work for a 22 years old:
Also, Sofro’s friend Boris GOLTZ (1913-1942)
Lekeu’s violin Sonata is a massive work.
Da Scrib’s son Julian wrote a few preludes.
I can’t think of anything which has left a dent with me (except maybe the Reubke, but from what I recall of it it has a similar musical language to Brahms’ F minor Sonata, Liszt’s 1850s Weimar period etc which isn’t my thing) but two names which pop up are Carl Filtsch and Alexei Stanchinsky. Filtsch was Chopin’s star pupil and died aged 14, but already seems to have left quite an impression on Chopin and Liszt. I’ve heard some of his compositions which I remember as promising for such a young composer, and where you can also spot the seeds of his musical personality which I remember (perhaps wrongly) as somewhat reminiscent of Henselt and Weber.
Stanchinsky I recall almost nothing of, except that friends I collected scores with in my teens and early 20s thought his works were very promising. Turn-of-the century character who died in his 20s, I associate him with Scriabin… but I don’t remember why.
Thanks mate, I remember when you uploaded this and I liked it. I don’t remember how I reacted to Goltz’s set of Preludes, so will try that again…
One I already know and is a must-hear IMO is Alexei Stanchinsky (26). Ekaterina Derzhavina recently recorded his piano works.
Oh yeah - also Tausig, of course.
Yes, Stanchinsky - I totally forgot about him!
I think I have one CD with early recordings of Samuel Feinberg where he plays a short piece - I remember it didn’t make much of an impression then but I should listen again.
Wasn’t there a baroque guy who died very young??? I can’t think of a name 
Alain, too.
Right - Burgmuller (of the Norbert kind).
Thomas Linley. The Brits rave about him.
Gideon Klein. Just throwing them out there. 
Kissin recently played some of his stuff.
Edna Stern played his sonata.
Pergolesi was the baroque composer I thought of earlier. Primarily of Hamelin fame here perhaps, but also the Stabat Mater.
Randomly…
Pergolesi -> tuberculosis
Filtsch -> tuberculosis
Chopin -> tuberculosis
Reubke -> tuberculosis
Weber -> tuberculosis
