I don't know if it's just me, but I sometimes hesitate to comment on a story or respond to a question if it's been around for more than a few hours, and the conversation has died down. But then I sometimes get notifications that someone also responded to a story from like two months ago, so I don't know. How old does a post have to be before you feel weird about commenting on it?
I look through the stories in groups from time to time, and if one catches my attention I'll comment on it. I don't find it weird at all, it's obviously still there for a reason. Besides, by commenting, a whole new discussion could arise for it along with fresh new replies and perspectives.
It depends. I often check a persons older posts if I don't know them before I comment on a new post. Even then Ill normaly just heart anything that I found interesting.
I comment on old stories all the time, because I appreciate when people find old stories of mine. I figure if they're still up and the poster's avi doesn't indicate that they've left the site, they'll still get the message.