If one wants to be pedantic, then really, very few visual diaries or art journals qualify for their title. Few artists seem to make daily entries, which to my mind is what a journal or diary is all about. Not criticising here. Just idle musings from an infrequent journaler.
Most journalers also keep sketch books in which are ideas for future paintings or just something intriguing such as the way the light hits something and generates fascinating colours. I wonder how the daily journalers/sketchers etc., find time to do all that, probably keep a sketch book and produce some larger works too. Obviously they are better organizers of their time and more highly motivated than me.
A delightful online community is rapidly building, centred on visual journaling (which seems a better "catch all" title for this phenomenon). You can find them at Wet Canvas Art Journal forum which is a little tricky to find as it is not included in some of their Forum links, but you can find it listed on the Wet Canvas home page if you lose the above link. Another delightful Forum group based at Yahoo Groups is Everyday Matters
Do check out Roz Stendahl's site Official International Fake Journal Blog where she blogs about her fake journal and has been running a competition for the month of April for other's fake journals. The concept challenges fellow journalers to keep for one month, a visual journal about the doings of an imaginary character. So much fun for many participants. You can also see some of these fake journals on the Art Journal Forum at Wet Canvas.
Mariza at Marizatera.com has some good info to get you started in the visual art journaling scene. I love her advice: "Don’t get caught up in what kind of journal or tools you need (obsessing over this stuff is a form of procrastination); it doesn’t have to be perfect".
A lot of journalers are using "altered books", which are old, bound books ranging from accounting ledgers to tired novels, and painting, collaging and drawing over the top of the old written upon or printed pages. Looks great. Check out a Google Images search for "altered books" - there's some mind blowing stuff there, but amongst the unbelievably complex creations are some not quite so ambitious journal pages that might inspire you to try using an old book as your journal base.
It seems that part of the pleasure of this art form is in sharing your work on the forums or through personal blogs like this one. Flickr has a very active Art Journal pool of member's journal pages too.
It doesn't seem to matter one jot how young or old you are, if you have artistic talent oozing out your ears or have never picked up a paint brush in your life; whether you are a trained artist or an absolute novice at anything arty. The catch cries for visual art journaling might best be "anything goes" and "enjoy what you do and share the joy around".