As such there is no official rule about answering questions within a certain time & not answering old questions. There may be lot of questions which have gone unanswered, so an informative answer will add content to an otherwise empty page. Further, we do approve answers to even questions posted a year ago if these answers are providing some additional, useful information. Even correcting incorrect answers that may have been given earlier are permitted. What is not advisable to do is to post an answer with information that has already been provided in great detail. If it is found that a member is deliberately posting answers to old questions when further answers are not really required, then the member's posting permission will be removed.
When people come at you with their worst, you should come at them with your best (advice given to Selena Gomez by her mother, quoted in Time magazine.)