LotR -- Book 5 - Chapter 01 - Minas Tirith

[Lathriel]

You can see that Pippin hasn't fully matured yet because if he had he might have thought twice before he offered his services to Denethor. He still lets his emotions guide him, but thet don't overtake him as much as they used to before.
He is also losing some of that innocence that he had back in book 1 or just some of his ignorance which led to curiosity and drove him to do silly things like throwing the rock down the well in Moria.