I agree with most of the suggestions above. I personally would rather there not be a culture of editing titles by putting [SOLVED] in them (we all know how weird things get when the OP changes a node and later replies look out of context).

If there is some sort of flag implemented, which I think is a good idea, then one of the states of the flag should be "this node is too old to have been flagged" and this clearly noted in the Search results and such.