Indeed, it is OK to answer old questions. And it does not at all matter whether the question has an accepted answer. You should only expect to be voted down if your answer is generally misinforming, inappropriate, or simply not up to accepted standards. Normally, the worst case would be no vote at all.
If you post an answer to a question over sixty days old and you are able to garner five up-votes from that answer, then you will be awarded the silver Necromancer badge. If you fall into the happy situation of being the first to answer a question that is more than thirty days old and you gain two up-votes for this, then you receive the bronze Revival badge, which is even rarer, as most questions get answered within the hour.
Thus, you are encouraged to provide answers to all questions, however old they maybe, and especially if you have a fresh perspective or new insight into the matter at hand. I often dig deep into the site looking for old questions to answer and I recently got rewarded for one such answer. Even then, I had to wait for almost three months for the reward.
In closing, I will point you to this evergreen question. If you feel so inclined, definitely post an answer, and you may well be rewarded for your efforts!