i am! but i see a fair amount of people just starting their careers, students, etc. as well. and, based on some of the comments ive seen, i think there is a lot of young folk. most of my students are active, or at least browse, HN. they are mostly 18-20.
i took a wild guess that ~30 would be the average. maybe 35-40 is closer. either way, i think my point stands: 30 years seems too long to be classified as a fad.