How old is a 20 hp mercury thunderbolt with serial number 7139300?

To determine the age of a 20 hp Mercury Thunderbolt with the serial number 7139300, you would typically need to consult the manufacturer's production records or a reference guide specific to Mercury outboards. However, Mercury outboards from that era have serial numbers that can often be decoded to find the year of manufacture. Without access to specific databases or records, it's difficult to provide an exact age, but generally, Mercury outboards from that range were produced in the late 1970s to early 1980s.