I believe that you can receive rewards for answering old questions. 

For example, the below question on bandwidth was made 44 days ago.  One answer made 2 days ago and received a 25% upvote from musing, so quite a high upvote:

https://musing.io/q/tsnaks/pkbllpls5?r=answer

However I would expect that the answer would need to be strong to attract rewards, particularly if there were many previous answers. I always think you need to be adding something new or providing the most interesting / the most factual / the most engaging answer. This is more difficult the more answers there are for a question.