While this is the oldest box I haven't recycled yet (I've recycled everything except Bionicle canisters, which up until recently were used as parts storage), the oldest set I still have built is a RoboForce vehicle that is sitting on my shelf (gathering dust).
The Exploriens are my favorite LEGO Space theme, but I suppose at some point in my childhood I felt the need to take this set apart but was never compelled to do so to the RoboForce one. At this point I don't really feel like I should, if only to keep a bit of my childhood around (as well as a variety of figs).
Also unearthed was a tub of Bionicle things I didn't miss: Piraka spines, Kanoka, Rhotuka, Kraata, Zamor, and some torsos from 2008 and on. I reorganized my Bionicle collection late last year but never said, "huh, I wonder where those Piraka spines are." But, I know they have building potential. (I wonder if the Rhotuka and Kraata are worth anything in my BrickLink store....)
Oh and I found the LEGO Club BrickMaster CD with Bionicle comics #1-25 on it! Now I can reread most of the story (seeing as how I have no idea where in my house the physical comics are, despite now wanting to bag and board them).
Just another day in the life of an AFOL!