>I was surprised because I thought the crowd here was mainly young.
Maybe there is something about being +40 that compels one to disclose one's age?
Good on you to say "most respondents" rather than "most users." This voluntary response poll will tell us nothing reliable about most users of HN, even if more than half of all users respond (which is not particularly likely). See
http://www.edcallahan.com/web110/lectures/litdigest.htm
(you are probably familiar with the example, but this is posted for onlookers) for an example of a poll with a huge sample size that still got the wrong answer.