With this approach you are going to make it difficult for new writers. You could also consider length of answer, grammar, known vocabulary. Quora seems to also

With this approach you are going to make it difficult for new writers. You could also consider length of answer, grammar, known vocabulary. Quora seems to also seems to rank recent answers on top for a while until it reaches a view limit. So probably takes rate of up vote as upvote/view count. Instead of taking age of answer you could consider view count . Have a look at RankSvm if that helps. It would require good amount of data though .