Depends on the subject, how old it is, what you have to say, etc..
You probably did nothing wrong, it's just that some annoying idiots feel the need to fill every thread with spam/junk and should have their post count reset every time they do.
Downside of responding to old threads is that others have to read everything again and aside of that might not notice the age and respond on things that are no longer at hand or the original writer left and will no longer read it.
So if something is already a few month old I think you might be best of starting a new topic, you can then always include a link to other topics you found on the subject and/or quote things you need. Again it all depends on the situation at hand.
p.s. Battle Grangers are cool.
