It's really up to you!!
This has been discussed many times before, and it's been made clear that there is no policy saying you can't post a comment to old(er) threads.
As someone already said, sometimes it's good to see an old thread come back active again ... and it lets others to see something they might never have seen in the past.
Similarly, it's important to remember, that some unsubscribe from older threads. So if you are directing your comments towards someone, they may never see them. Likewise, they may not get the PM if no longer active on the Board.