I doubt this will impress, so much as amuse. But I'm pretty sure the oldest thing I've written that's still in use is a series of popular scripts used in various Warcraft 3 maps and mods. Written circa 2003 if memory serves. Mostly they focused on adding RPG-like elements to gameplay: camera lock on individual PC units, unit swaps to "upgrade" character classes, and so forth.

At the time, JASS (Blizzard's scripting syntax) was largely procedural, so this was some brute-force work that was a real bear to make usable across different mods.

Still playable, though, and still enjoyable.