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.