Imagine you'd gain additional rep points for answering old (unanswered) questions. For instance one of these models:
- 1 additional rep point if the question is older than 3 month
- 1 additional rep point for each month of time difference in an unanswered old question
This would lead to a higher chance to get old questions answered - surely many people would jump right to the last page of the unanswered questions. I believe having all (including old) questions answered is a design goal of this site.
Still, I believe this would be a bad idea. Old unanswered questions are usually in that state for a reason. Poor wording, poor content, you name it - these questions would usually (exceptions!) be better off not asked. Also, there is a fair chance that the asker is not paying attention to the question anymore, and even if he is, it is unlikely that he still gets any benefit from having such an old question answered.
Another idea is to give less reputation points on answering old questions. I don't see how this would help at all.
After all, this is not a proposal but an idea, that (apparently) was not presented yet. Maybe it is of some use, so I put it up here.