I think the lack of under 25s watching is down solely to the competition. I think the under 25s are the most likely to pass on Trek in favour of something else (FF6, Gatsby, IM3, Hangover) because it is not their first choice movie. If it is released in amongst less competition the percentage would be up like ST09 was.
Most under 20s will have been exposed to Trek TV shows from channel hopping. They probably think it looks dated and likely does not appeal to the majority of them at all. They also might compound that by catching a bad episode. So they wont give the movie a chance.
At some point the concept of Star Trek will become as dated to people as Flash Gordon from the 1930s does now. It will take a bold producer to really shake up the concept way beyond what Abrams has done to make it relevant again. Times change.