My mistake. It's not viewable from Preferences. Go to "Get Info" from the drop-down File, and then a window will pop up. The tabs are:
Summary ~ Info ~ Video ~ Sorting ~ Options ~ Lyrics ~ Artwork
When you click on Sorting, you'll get that song's information (i.e. track name, album name, etc.). You can then enter the criteria you want (e.g. year) in one of the "Sort" boxes (though there's actually no "Sort Year" box; I use the "Sort Album" box). The default for iTunes is to sort alphabetically by artist first name, and then alphabetically by album name. This can be changed with the Sorting feature. I had to play around with it a bit, but now my iTunes library is sorted by artist, and within artist by year.
I hope that helps some to clarify!
Late, you're a genius. I've done a provisional update using this method on the Various Artists comps and one of the Mosaics and it works like a charm. Many thanks. Still don't understand why Apple changed it, though -- it was fine as it was!