Well, mine break down into two categories. Kits I bought but never started and kits I started but never finished.
The oldest kit I bought and started was the original Tamiya M2 Bradley. I bought it while in college around 1986 and never got around to finishing it. I left it at mom's when I went into the Army and have since retrieved it (about 18 yrs later). I still haven't completed it, but I did buy another one and built/finished that one.
The oldest kit I bought and never even attempted to build was an Italeri CWT gun tractor truck. I bought it in 1988 when I was stationed in Germany. Never started it, and still have it today.