Well,
a site-wide poll of hybrid owners from April 2005 had 45-59 tied with 28-35 (20 votes each) for the lead. That's with 78 votes.
the TCH poll said the average age was 46-55. That's with 66 voters.
The HCH Poll said the average was 35-60. That was with 74 respondents.
These samples are so small that the margin of error is huge, but an average age of 55 wouldn't be totally unbelievable.
Bear in mind that someone 55 today would have been 17-18 and paying for their own gas during the 1973-74 oil embargo. It doesn't take a genius to learn that lesson.
Also, the higher income plays a huge part. Hybrids may only run $3000 or so above their non-hybrid equivalent, but how many people that are just out of college and paying off that debt were going to have $21000 to drop on a Civic EX? I'd expect them to buy something cheap until school is paid off, but that's admittedly based on my own experience.
Finally, at 55, your kids have moved out (hopefully) and you can get something a bit smaller.