Just saw this post,, and your question reminded me of a video with Peter that has done plenty of Waves tutorials and was a huge part of creating Waves GTR 3,,,
So, here is the youtube link,,, of him explaining in a bit of detail of making a vocal sound younger,,, Funny guy and a history of recording well known artists,, He actually makes up words explaining how to help a vocal sound younger, and how he gets there,, lol,,
It impressed me enough to remember his example lol,,,
younger vocal example,,