I started it at the end of March 2013, in 1.5.1, so it will be 3 years old in a couple months. I haven't exclusively played it for the entire time though, first for about 6 months, then I played on various other worlds for about a year before playing on it again for another 6 months or so - except that time I modded a copy of the world with new caves and biomes added by one of my mods; when I recently started playing on the world again (since this past September) I started on an earlier copy and discarded the modded version (that said, the world as it is isn't entirely unmodded; I removed naturally generated torches from mineshafts and also prevent mineshafts from generating in areas of high cave density, reducing them by about a third overall, but still much more common than they are in 1.7+).
Here is what the modded version of the world looked like (a few areas have been explored twice, the modifications I'd made to caves did not significantly alter them, mainly by adding occasional larger caves and ravines, but here I mostly explored the area under the ocean to the north, in contrast to my current exploration to the south):
(of interest, there are no visible chunk borders because I added new biomes so that they only replaced biomes with similar characteristics and/or I made sure that biomes like mesas did not border already generated chunks; the worst that I saw was a few block drop/rise which was easy to smooth out with a shovel. The landmass near the top with a Mushroom Island doesn't exist in vanilla and was the result of adding more landmasses to oceans by making Mushroom Islands 15 times more common and making 14/15 of them other biomes)
Also, here are renderings of the world from after the first stretch of playing (I manually removed unexplored mineshafts from the cave map); even at this point it was already larger than any other world I've had since:
By comparison, this was one of my modded worlds (none of these are necessarily resized in proportion since I pulled them from various threads):
(this is basically what a 1.7+ world would look like without climate zones; I added many of the 1.7+ biomes as well as some of my own; all of my worlds have been in 1.6.4 or earlier. Also, this world used the same seed as my first world; the ocean seen near the bottom is the same ocean you can see near the bottom of the maps above)
(what's not obvious here is that some caves and ravines were MUCH larger than anything you'll ever find in vanilla. Near the top left is a stronghold, separated from the caves and the only area I explored underground away from my base early on; even while branch-mining to get resources I only explore any caves I come across enough to see if they lead anywhere)