I'm 35.
I'd guess it's a broad demographic these days, with a pretty even mix of high school up to 70+. Probably most concentrated around 30-55 white males. I've been an infrequent poster/lurker (always a different name) for 15+ years, starting in college. Seems to me like most of the pros/active college runners got scared off in the mid 2000's, but those of us who started earlier tend to hang around. Not that I ever approached anything but sub-sub-elite.
I'd agree that the civility (though not necessarily quality) of the board has improved significantly in the past few years. This does seem to correspond to the Brojos maturing and taking their journalistic and business "responsibilities" more seriously. This seems to have led a new crop of younger runners back to the board. Believe it or not, it is also less sexist than it used to be, though most women were (justifiably!) scared away years ago.