Oldest MOC you have still intact.

[Natalya]

The Czech Death Car is probably the oldest dating back to 2002, but I completely rebuilt it about two years ago, so if you don't want to count that then the Venus is the oldest MOC I have. It got renovated like 3 or 4 times until its present incarnation, but it is the oldest MOC where most of the main superstructure has remained unchanged, dating back to 2006.

[Arkbrik]

My oldest MOC would be the Bloodstained Serpent, built in 2009. My MOCs rarely stay built for very long, the only ones I keep around are my armies.

[muffinman42]

Working vacuum engine.
Works by the same principle as an oscillating steam piston, but as plastic melts it runs off air. It's a bit broken as i've been modifying it and haven't repaired it.
It's at least 2 years old.

[Tzan]

The Bumble Borg, its been taking up 3/4 of my living room for 3 years now.
Every time I try to take it down the minifigs inside defend it and prevent me from dismantling this monstrosity.
It would probably take me a month to tear it apart anyway.

[RunsWithLegos]

Bumble borg? do you have any pics tzan?

[Tzan]

Sure, we have pics.

viewtopic.php?f=8&t=7443

[RunsWithLegos]

my lord! how can you even store that in your house!?

[IVhorseman]

I've got crap lying around still constructed since 1997, but no pics (yet).