I know it is not classic rock, so it is outside the parameters of the question, but I was genuinely surprised when I was putting together the 26 September birthday list in the GDT to learn that George Gershwin was only 38 when he died.
In the classic rock genre, I would have to place Jimi Hendrix at the top of any list, but I agree with most of the ones you guys have cited; at least the ones I'm actually familiar with.
EDIT TO ADD: Jim Morrison, died aged 27 years old.