To determine the age of an Iver Johnson 16 gauge single shot shotgun with the serial number FBCU, you would need to refer to Iver Johnson's manufacturing records or a specific serial number dating guide. Iver Johnson produced firearms from the late 19th century until the 1970s, and the serial number can help pinpoint the year of manufacture. Typically, Iver Johnson Firearms from this period can be estimated to be anywhere from 50 to over 100 years old, depending on the specific production date associated with that serial number.