From time to time I find an old question and feel the urge to add an answer or comment. But is it really worth the effort? People's attention have moved on and one's contribution may simply be lost or ignored. On the other hand, if one has a worthwhile answer one is contributing to the body of knowledge that others will later find when doing searches.
Definitely. People will find these questions and answers via Google having no reference to date or time when it was asked just looking for an answer.
The system is designed to promote this through the necromancer badge which is awarded for:
Answered a question more than 60 days later with at least 5 votes. This badge can be awarded multiple times.
To this effect on SO I have answered questions over a year old that have then become the most popular answer and updated old answers with new links.
On the other hand, if one has a worthwhile answer one is contributing to the body of knowledge that others will later find when doing searches.
Indeed. I've actually learned a lot by reading older questions on the site - many of which negated the need for me to ask a similar question. If you have something worthwhile to contribute, don't let the question's age put you off.
Answering old questions is not really worth it, if you are in the hopes of getting good reputation points out of it. The old question does not appear on the front page for people who have it sorted by 'newest' and so many members of community don't even see that you've posted a new answer. And for those who get to read it via search results are surely prone to read just the question itself and a couple of top voted answers, which generally leaves your new answer out of the loop because it does not have enough votes yet.
Th Necromancer badge is nice, and it only needs five votes to get, so I'd think that is not too hard to achieve.
I have two Necromancer badges and two Revival badges, because I like reading old content and occasionally I post an answer too. More likely my answers haven't been so very cool to get voted up, but sometimes someone has given an upvote and I get those badges ;)