When I am looking at questions, I see the related questions over to the side. Recently I have been clicking on them and adding my own two cents. Then I realize that the questions is a few weeks or months old and the original person may never even see it. Is it considered valuable to answer these sort of question (for the sake of future visitors?) or is it simply vanity and I should pass on them after a certain period of time.
I would definitely answer the old questions.
For one, a new answer "bumps" them to the top of the front page. Frequently, a new answer to an old question will revive a question and give it a breath of fresh air including many new insights.
Stack Exchange also encourages this through the necromancer badge.
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, since anonymous and low rep now have the ability to submit edit suggestions, I would "punch up" the question through judicious editing when answering, to maximize the chances of people seeing both the question and your answer.