The Official Amazon Kindle Forum has a thread called the "Average Kindle Owner's Age." Someone counted up all 1,387 responses and found that 70% of respondents were over 40.
Keep in mind, the data is by no means scientific. There were no controls in the polling process, meaning that it's quite possible for the average Kindle Forum reader to be over 40 (or something exactly the opposite), while the average Kindle buyer is of a complately different age group.
Still, it's a staggering number that's tough to completely write off as fictional. Kindles are obviously successful in their niche, but for most gadgets to take over the mainstream, they need to have a level of universal appeal that, sadly, supports tweens, mid-20s jerks such as myself and all those 40-year-olds who just CLAIM to be in their 30s during anonymous online polling. [Kindle Culture via Gear Diary]