This honestly makes me pretty curious. I thought the 25-34 range would be larger, especially compared to other sites that seem to have a much less professional atmosphere(Reddit, etc). Hacker News certainly isn't some bastion of intelligent thought and discourse, but it's definitely above average in that, which makes me wonder if the 18-24 year olds are just keeping quiet in the comments, if they're really that smart…
HN's issue appears to be that it promotes what younger people view as an aspirational life style. College grads and young techie types will read stories about life at Google, joining startups that become large businesses like Facebook and will think to themselves "Yeah, I want a piece of that!"
In reality, HN can be poor for tech discussion and there are some absolutely insane comments on here, to the point where parody accounts like "Shit HN Says" have become very popular. It's an aspirational community with the best intentions, which is fine, but it won't bring the best in business or battle-hardened sysadmins to the table. You're far more likely to see a 22 year old college dropout gushing about how MongoDB is the future than someone that has succeeded in the environment that many people here aspire to join.