I am terrible at finding things on HN. There is a search function at the bottom of the site and there have been surveys performed in the past on site that asked for things like age (and sometimes other things, like religion). I have provided all the raw data I know of regarding gender on my blog (linked in my profile, as you likely know). I know of nothing formal, rigorous, etc. But I believe I have seen more than one age survey over the years.

I have been here on and off something like 6 years. This is just what I remember seeing off the top of my head at various points in time. And my memory is certainly not 100% reliable (it used to be a lot better than it is these days, nearly but not quite "photographic" for some things, but that was some years back).