I'm close to 42 and I've been here for 15 years now (as a reader more than as a contributor); when I joined I certainly didn't expect to be here for that long. Testament to the quality of discussion, although it has gotten a bit partisan of late. I was on Slashdot prior to that for about 10 years. What was most appealing about HN was the information density, and the discouragement of jokes. I guess if you distil a UI down to the basics (HN just being pure text) there is a certain timelessness to it. Slashdot soon felt bloated and stuffy and I never went back.
Having said that, the fact that the 16-20 and 21-25 is so under-represented doesn't bode well for the future of the website.
Edit: I think 36-45 was the largest group when I started writing the comment. It isn't now.