Young people were affected to a large degree by close association in schools, churches, playgrounds, battle conditions, etc. The elderly have compromised immune systems due to advanced age while the middle-aged carry more antibodies and were more isolated in many cases.
The spread to Alaska is the most remarkable aspect of the 1918 flu; a Dr. Johan Hultin was reported in 1998 as having begun a study decades earlier on well preserved tissue from a victim in Brevig Village where 79 of 80 died.
This link dated 2002 is the most up-to-date story I can find concerning his work, it is worth a read: http://www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/2002/02/17/CM40502.DTL
An amazing story and an amazing guy. We need more like him.