There are several posts indicating that, generally, yes… it is considered appropriate to flag old posts that should be closed:
- Is flagging to close old questions best practice?
- Should I vote to close old questions?
- Should I flag old questions?
- Closing old questions that didn't show enough effort but were answered?
I would image that the key is that one should flag obvious cases, so that someone reviewing the flag has an easy decision to make.
If I am familiar with a particular tag, such that I can readily identify programming-related vs too-broad "I have no clue" type questions… it seems like it might be helpful for me to spend a few minutes periodically flagging a number of old posts on that tag/topic. It seems there must be an overwhelming volume of questions to deal with (on SO, in particular).
In terms of "taking out the trash", what is in shorter supply on SE, users who can appropriately flag posts for closing, or higher level reviewers privileged enough to do the actual deciding/closing?