Into the Wardrobe

I've seen lots of explanations that purport to explain why it is that both Tom and Treebeard are claimed as the oldest living being on earth but none of them make much sense to me. i think that Tolkien made a mistake here and that's unusual. Did he ever have anything to say on the subject?

Whilst we're talking about mistakes, here is another one. When the three hunters are approaching Isenguard they find some slain orcs bearing an S symbol. Gimli says 'S is for Sauron, that is plain' and someone replies that Sauron does not permit anyone to speak or write his real name. And yet at the council of Elrond when Gloin recounts the story of the messenger from mordor coming to Dain, the messenger refers to Sauron by name. More to the point, before the Black Gates the Mouth of Sauron refers to himself specifically as the 'Mouth of Sauron.' Why the discrepancy?

Ummmmmm... but in that case, Legolas would have believed Sauron to be his right name and that brings back the original question.. why did he say that Sauron did not permit his name to be spoken when he does?