I think the biggest reasons we don't see more younger folks in this hobby are as follows:
-Today's instant gratification society. "I want it and I want it NOW!"
-No time to build kits. We (at least the reponsible ones) are focused on getting on with our careers and starting families. I work full time but also part time at my local hobby shop as mentioned. I buy models now because I get a good employee discount but they go into the stash usually for lack of time to build them
-In establishing a career, it is often necessary to move. We all know assembled models are a pain to move. Younger people tend to move around more before settling down someplace.
-Once it is built, models just "sit" there. Radio control is a much more popular hobby because they can be "played" with.
-History and its impact on our day to day lives is not recognized. I mostly taught myself through my own research all I know about World War Two. I also developed my own methods and applied learned methods from reading books and message boards like this on how to build model kits. People seeking instant gratification over patience and perseverance would never give much thought to doing this, let alone spend time making miniature models of it.
I got hooked by watching specials on the 50th anniversary of Pearl Harbor on TV when I was 7. Growing up, I always heard about how my grandfather served in General Patton's Third Army and went from Normandy (after D-Day) to the Battle of the Bulge, and on into Germany. This family connection plus my interest in all things World War Two dovetails perfectly with my like of building things with my own two hands. It also helps me understand the design of the ships, planes, and armor utilized and I also see it as honoring and respecting those soldiers and sailors that operated them.