Gandalf's comment about Frodo

[Galadriel55]

Originally Posted by Lotrelf

You're right, there! Gollum's case is different. He had surpassed the normal age of his life. But Bilbo would look younger than he actually was. He couldn't appear older than his actual appearance. How come Frodo looked older? The Ring played a part is true.

Frodo also looked quite young for his age when he left the Shire, but his internal active battle with the Ring wore him down during the course of the few following months probably more than even the physical trials of the journey. I think that is what Zil was referring to.