Some questions may not be on the first few pages anymore but I still want to answer them. Maybe I think that I know an answer to them.
Is that OK as per site rules? Can I answer "old" questions that do not have any answers or a best answer yet?
It's not just 'OK', it's actually recommended.
Answering old questions has many pros.
It will get bumped to the frontpage. Getting more views and your answer possibly getting upvotes
You answer the question for anyone who comes later with the exact same question.
You show others the question, who might answer it.
You might get a badge for answering an old question
The user (if still active) will get a notification of new answers to his question