Answering old questions is not only welcome, it's encouraged!
Ask Patents is, as you know, a community-run site, just like all the others in the Stack Exchange network.
From the About page for Stack Exchange,
Founded in 2008 by Joel Spolsky and Jeff Atwood, the company was built on the premise that serving the developer community at large would lead to a better, smarter Internet.
Now, that's saying developers because of the flagship Stack Overflow. But if we jump over that, we get to the main purpose for the network.
We want answers!
Around 96 million people came to Stack Exchange from across the net in the last month alone, looking for answers to all sorts of questions, and as just about anyone who's ever tried to look anything up can tell you, there's little more frustrating than having a problem, finding someone online with that exact same problem from years ago, and seeing no answers to it posted.
New answers to old questions may or may not get accepted (since, particularly here with the current state of Ask Patents, many users may not come back after a day or two), but they still play a great role in improving the quality of the site as a whole.
Even aside from the simple principle of the thing, consider a user who has a question about patents, and comes to find that a significant majority of our questions got at least one answer. That'd be quite the incentive to post a question.
As for moderation, we have a few tools for that.
The most obviously-relevant one is the Late Answers review queue, accessible currently at 350 reputation points. This queue is populated with late answers to questions, and helps to weed out some spam and non-answers ("thanks!" or "could you clarify this?") by users who don't yet understand our format. Review queues aren't a great place to get feedback on technical accuracy, but that is somewhere that late answers will show up.
The other thing late answers do is "bump" questions to the homepage. That's where you're most likely to get technical feedback, and it's another line of defense against the aforementioned spam and non-answer posts. This also gives users an opportunity to vote on your posts, meaning you aren't left in the dark just for answering something that not everyone is already looking at.
As far as technical accuracy is concerned for any of your posts, though, the best defense is a good offense. Back your posts up in them. Find evidence to support your statements: the USPTO has great resources about the patent system, just as an example. If you can quote some official documentation (including, of course, attribution), as well as summarizing it in your own words, you'll get confirmation that what you're saying is correct, and you'll end up with just that much better of an answer.
I don't mean to single my own answer out, but just as an example of citing an off-site resource, you can take a look at my answer to "Another purpose of patents - Public record?. It's a small citation, but it goes a long way, in my albeit biased opinion, to backing up the post.
The one thing I would prefer you not do is, as I'm sure you know, guess at answers. There are good kinds of subjective and bad, and sometimes what you view as an answer might not agree with what someone else does, particularly for prior-art-requests, but in general guessing isn't the best way to answer questions, particularly on a site of this size. If you come across a question that you don't know the answer to but would like to, you can always post a bounty so that someone else may notice and answer it.
But if you're more worried about the quality of your answers, or how well they fit the standards of the site, asking on Meta is a great thing to do.