It's easy enough to find lists of inventions created by ancient cultures, but I can't find any listings of old inventions for which we know exactly who created the prototype.
Far from the oldest known I imagine, but this is one of the few times I get a chance to bring up an interesting figure in history like Hero of Alexandria.
Born in 10 AD in Roman Egypt he is the inventor of a number of ingenius devices such as the vending machine, types of syringes, and various measurement instrumentation. He built mechanisms for a number of contemporary uses including making sound effects for the theatre, stand alone fountains that worked on hydrostatic pressure, automated mechanical plays, programmable weight driven carts, and early steam engines. He pioneered early optics, pneumatics, hydraulics, mathematics, automation, and physics.
The list goes on. I only know about him because his name popped up briefly in half the textbooks I had back in college when I was studying instrumentation and automation.
I've always pictured him as a real life version of the medieval gadget inventor trope, peddling away at some wooden construct of pulleys, paddle wheels, bellows, and gears to complete some trivial task. That's probably taking too much creative license though.
This is a tough question...the Encyclopedia of Cryptography mentions one Aeneas Tacticus (300s BC) as being the first to develop relatively secure means of military communication. Polybius's Histories describes how he invented the hydraulic semaphore, which was a sophisticated device/system capable of transmitting messages in combat.