Did some further digging. According to this guy new OEM rear rotors are a little more than 18mm.
5th Gen Rear Rotors:
Nominal Thickness: 18.034mm (0.71")
Minimum Thickness: 16.002mm (0.63")
Outside Diameter: 312.166mm (12.29")
So it seems that my rear rotors are relatively new. Mine measured just a 18mm.
5th Gen 4Runner Brakes Upgrade (for 4th Gen 4Runner)