Why is Sanji from One Piece described to be the second son of a family when he actually is older than Zoro?

Zoro and Sanji are actually the same age. I'm pretty sure he's described as the second son because Zoro is much more watchful and protective of all of the crew, very much like an eldest brother, and everyone relies on him. It may also be because he was the first to join Luffy. That's just how Oda described them though...