I often wade fairly deep into the Unanswered Questions list, and after the first few pages I start seeing questions that were posted months ago. If there has been steady activity since the original post, I have no qualms about jumping in with my own answer/comments. But often these older questions have had little or no recent activity. Worse, the original poster often seems to have abandoned the question as there is no evidence that they ever responded to answers or comments, and obviously no answer was accepted. In these cases, I can't help wondering whether I should bother adding my own answer or comments, even if I think I can contribute meaningfully. What's the general feeling on this? Perhaps if there was more obvious encouragement to answer older questions it would help. I'm not sure what that would look like, but maybe an extra point or two for contributing to older questions?
I think yes -- if it's a good answer, I will always upvote it, regardless of how old the question is.
However, this assumes the original question is interesting enough for me to click on in some way -- if it's a really bad question (title, etc) or an extremely obscure topic. However, this can sometimes be fixed with editing..
Thus, if you have the rights to edit (2k rep), I would "punch up" the question as well when answering to maximize the chances of people seeing both the question and your answer.
I don't know if you should specifically award this with rep... However I do get what you mean and have only this to say:
People with questions come looking for answers on Super User and might have exactly that same question. If you have contributed by means of a useful answer, those people may vote up your answer (As well as others, since it is shown on the front page after an answer has been added). In addition, it will help those people and they won't have to create a duplicate question which will then be closed as a duplicate! Hurray!