According to the Star Wars: Episode 1 - The Visual Dictionary factbook,
Standard Cybot Galactica protocol droids have been in use for generations.
Most of C-3PO's structural framework is more than 80 years old.
He is, however, a uniquely built droid (and the first of his kind) in the sense that he's made of scavenged parts from a range of protocol ...
C-3PO is a "3PO Series" droid manufactured by "Cybot Galactica", which Anakin has assembled from spare parts.
Until the release of The Phantom Menace, the Expanded Universe had him as a regular droid who had been manufactured 112 years BBY.
Between The Phantom Menace and the "Legends" retcon, some of the discrepancy was ...