Knock yourself out.
Don't bump a thread that's less than a day old just to get responses to it, but if you're replying to an old thread, that's fine.
You may want to mention in your reply "I know this is an old thread, but..." to give people reading some context.
Another alternative is to start a new thread on the topic, and link to the old one and say "this was discussed six months ago here (link), but I wanted to reopen the discussion here because..."
But if you're just replying to a thread that's a few months old, but think is still relevant, feel free to reply in it.
However if it was a controversial thread at the time though (mod warnings and such), best to leave it dead. ;)