As far as I'm concerned, there's nothing particularly wrong with finding an older post IF you have a very similar question or issue. There's nothing wrong with adding new, relevant information to an older thread when it is found to be the most appropriate location. Google often pulls those threads to the top of searches, so keeping information on certain subjects together makes sense.

The problem comes with actually going and answering an ancient thread as if the original poster is still searching and waiting for that information. Or providing your incidental thoughts on a subject long since retired, as if it were still current conversation. Or, adding old information when there are already current threads containing it.

tl;dr - If you're absolutely sure it's the best place, add on to a thread. But if there's any doubt, just start a new thread and perhaps link to the old one.