So many great composers and musicians die young.  One can only wonder "what if..." when considering their early demise. 
First, the classical composers who would have blessed us with more gorgeous music had they but lived longer.
Mozart  Died at age 35.  The acknowledged greatest natural musical genius who ever lived.
Schubert  Died at 31.  Rated by many as just behind Mozart in spontaneous creativity.  He was very productive and, had he lived longer, would surely have written much more beautiful music. One of my personal favorites.
Mendelssohn  38   Wrote wonderful melodies that were uplifting and graceful.  If you've ever watched the movie "Breaking Away," you've enjoyed his 4th Symphony.
Schumann   46   Died in an asylum after a nervous breakdown. He is regarded as the incarnation of the spirit of Romanticism.
Chopin  39   Tuberculosis killed this composer of short pieces of intense, poetic piano music filled with feeling.  I enjoy listening to his work when I get in bed at night.
Bizet  36   Composed the opera "Carmen."  Nuff said.
Mussorgsky   42   An alcoholic, he was the most nationalistic Russian composer. Wrote "Pictures at an Exhibition" & "A Night on Bald Mountain."
Gershwin  39  I could cry when I think of what this great American composer would have written if granted a longer life.  In some weeks, I will listen to "Rhapsody in Blue" every day.  Then there is his opera "Porcy & Bess," not to mention all that great popular music.
Speaking of "popular music," so as not to slight non-classical musicians, the list of rock stars who have died early deaths is astonishing.  My list  below has 27 names, and they're just the ones I've heard of.  I will list the musician's name, age at death, and cause of death.
John Lennon          40          Murdered
Bobby Darin           37          Heart attack
Jim Croce               30          Airplane crash
Jimi Hendrix           27          Drug OD
Janice Joplin          27          Drug OD
Buddy Holly           22          Airplane crash
Bob Marley            36          Cancer
Frankie Lymon       25          Drug OD
Karen Carpenter   33          Anorexia
Tom Fogerty          49          Heart attack
Kurt Cobain           27          Suicide
Cass Elliot             32          Choked
Dennis Wilson       39          Drowned
Jackie Wilson         49         Heart attack/coma
Sid Vicious             21          Drug OD
Minnie Ripperton   30          Cancer
Sam Cooke            33         Murdered
Gene Vincent         36          Alcohol
Tupac Shakur         25         Murdered
Rick Nelson            45          Plane crash
JP Richardson (the Big Bopper)  24     Plane crash
Otis Redding         26          Plane crash
Richie Valens         17          Plane crash
Clyde McPhatter    38         Alcoholism
Ronnie Van Zant   29          Plane crash
Phil Ochs               35          Suicide
Jim Morrison          27          Heart attack
Fortunately, their music never dies...