The default Subsonic 4.6 behavior is to pass a list of MusicFile to main.jsp when you browse an artist. The MusicFiles represents either a single song or a directory containing songs (everything that's found directly below the path for the artist you clicked). The year from the id3 tag is only available for MusicFiles representing single songs, it's not aggregated into a MusicFile representing a directory. So if you have the directory structure Artist/Album/Songs, years will only be available when you get down to Songs, not in Albums.
I've modded MainController.java and main.jsp to get an artist view that looks like this:
http://dilerium.se/musiccabinet/#artistRadioif that's useful for you, you can have a look at
https://github.com/hakko/subsonic to see how it's done. The mod has a thread here:
viewtopic.php?f=11&t=10587