No, there is no time limit on voting. As long as a post continues to exist, it's useful for its score to be displayed to show people what other members of the community thought of it. It remains useful to be able to vote up or down on the post, just as it remains useful to be able to edit or comment on the post. The system allows votes to be cast on old posts for the same reason it allows them to be cast on newer posts. The reasons it's just as important to vote on older posts as newer ones are so many, I cannot reasonably list them all here. So I'll just talk about a few of them.
I regularly search the web for help with a problem or just to learn something I'm curious about, and find relevant questions and answers here on Ask Ubuntu. Many people do--it's one the reasons the site exists. (It's actually the main reason it exists; see below.) I readily vote on posts I find in this way, regardless of their age, though sometimes their age is relevant to what they are saying, for example by establishing that it is specific to particular versions. I vote--up or down--on old posts, based on the same considerations as newer posts. So should you.
It turns out that I am less likely to downvote old posts than new posts, but that is only because a significant fraction of my downvotes are cast on posts that are extremely low in quality or should never have been posted at all--such as answers that are not really attempting to answer the question, even partially, and were really intended as new questions, edits, or sometimes comments. Those posts are usually deleted--when I downvote them I usually vote to delete them as well--so there are fewer of them to be found among old posts. With that said, downvotes are not only for content where outright removal is justified, and I am fully willing to downvote old posts.
Prohibiting voting on older posts would have been trivially easy from a site development perspective. It was not done--probably because it would be equivalent to saying, "Old posts don't matter anymore." This would, in my opinion, have been a disaster, and if we were to adopt a norm of etiqette against voting on old posts, I believe that would have similar ill effects.
The tour explains the purpose of the site:
Ask Ubuntu is a question and answer site for Ubuntu users and developers. It's built and run by you as part of the Stack Exchange network of Q&A sites. With your help, we're working together to build a library of detailed answers to every question about Ubuntu.
When we are building a library of detailed answers to every question about Ubuntu, old posts matter as much as new ones. We don't delete old posts because they're old. We don't refrain from editing old posts when they can be improved. We don't refrain from voting on them, either.
It is fundamental to the operation of the site that people be willing to use their votes to mark good content as good and bad content as bad. Nothing about this is specific to new posts. The majority of all posts on the site are old. This becomes even more so with, uh, age. If you choose only to vote on new posts, nobody can stop you, and your votes still matter and help the site. But this is an artificial limitation. You should be willing to vote on any post, because that is what makes the site work.