Welcome Lievre,
There is nothing wrong with reading and commenting on old threads. Sometimes they can be a lot of fun..walking down memory lanes. What we frown upon are opening old threads like (just an example) a wedding with nothing more to say then: I really liked the flowers.
Like with any other thread, the best posts are those that contribute to the discussion. Our various trolls however try to up their post numbers by posting nothing more than a smilie, a ditto or a "I like that too" and to get a really high post count, they also open old threads not to read, but to add to their numbers. Which is actually the number one reason to get annoyed about opening old books.
Have a great and relaxing reading and if you want to discuss something, even something old, go for it. If we already have a better thread to discuss it, we will surely let you know..
