Hmm, that's not a big problem, you can comment on the old posts normally ...
You can't normally comment there as there's a prompt will display that you're replying a very old post and you should create another thread if necessary, somethin like that phrase. But yes, you can still comment if you proceed.
Thanks for guiding him and others, but seems like I've already explained it in my above post even though it's in between the words my words clearly indicate that scenario. For the rest seems like OP is not facing this scenario, there are some other types of prompts that you might face let me mention the most recurrent ones here with an edit.