I've seen old questions pop up (like 3 years old) because some users answer or comment on them.
Whether you should comment or answer is not a function of the question age, but only of the intended content of your post.
Is it proposing a solution? Is it stating that a solution is not possible? Post an answer.
Is it asking for clarification or some other improvement to the post? Post a comment. Comments are not the place for solutions, even though they may arise from discussions in comments. Comments are not subject to downvoting or editing by normal users, so they're outside most of our quality control systems. Do not abuse that to post answers in comments.
The question age really doesn't come into the picture in this decision. Also note that it's common here for questions about some particular version of Ubuntu to become generalised to multiple versions, so don't let that consideration stop you.
There is a Late Answer Queue, where reviewers review these answers and comments. If that late answer is not appropriate, non-moderators will flag them inappropriate, and moderators will take action.
Not really (if I am not wrong). If those comments are really useful to the question, the community will still appreciate them.
If you really feel that you know the answer to that question, you don't mind your answer get reviewed, the quality of the answer is very good, and there are no users suggesting that particular answer, feel free to answer/comment the old question. The community has the ability to judge whether your answer is a quality one or not.
