Several Blogads surveys have proven this myth wrong (30-40 dominates, but the distribution above and below that age group is fairly equally represented. See the survey results on this page), but let's do a quick poll and see what the results show.
Update: As of 2:30 p.m., 63 percent of respondents (over 2,000) are over the age of 35. Sounds about right. This myth that blog readers are all youngsters is clearly not true.