Yamaha puts a date of manufacture on its outboard motors. Sometimes, though, it seems outboard motor manufacturers are deliberately obscuring the date their products were built. That obscurity may be the result of the "fog of business" rather than a conspiracy. For example, Mercury Marine no longer maintains any information on motors built prior to their acquisition by Bombardier in 2001 and subsequent resale to Brunswick. Bombardier also acquired Evinrude/Johnson during this same period, so official records for those motors are often lacking, or are available only from unofficial sources.
Look at the registration information for your motor if you have it. The date of manufacture will be shown on your motor's registration documents. However, outboard motors are not required to be registered in most states, so registration information may not exist.
Look on the starboard (right) side of a Yamaha outboard's mounting bracket. There is a permanent applique that directly states the month and year of manufacture, as in "06/08" for June, 2008.
Inspect the nameplate of Johnson and Evinrude outboards for the model and serial number. Although Johnson motors--formerly built by OMC/Evinrude--were last made in 1998, many are still in use. The year of manufacture for models produced between 1980 and 1998 is coded in the sixth and seventh position of the model number. The ten-letter word "INTRODUCES" is the key to the code: the letter "I" equals "1," the letter "N" equals "2," the letter "T" equals "3." The number values of the letters continue to climb until "E" equals "9." The letter "S" equals "0." If the sixth and seventh position in the model number is "EC," the numerical value of "E" is "9" and the value of "C" is "8," meaning that the motor was built in "98" or 1998.
Use your computer to find information online for Evinrude/Johnson motors made from 1928 to 1980, since no official source of information exists. One of the unofficial sources, Marine Engine.com (Reference 1) lists the year of manufacture, based on the motor's horsepower and model number. Click the link for the horsepower of your engine and you will be directed to a list of model numbers made during a particular year.
Read the Manufacturer's Statement of Origin on Mercury Marine motors to determine the year of manufacture for motors built from 2001 to 2005. Look for the manufacturer's plate located on the transom bracket of Mercury Marine engines manufactured after 2005. The date of manufacture should be listed just above the serial number. Before 2001, back to 1940, another online search at one of the "unofficial" databases may be necessary. Two such databases are at East Coast Marine Service (Reference 4) and Master Tech Marine (Reference 3). The motors are listed first by horsepower, then by model and year of manufacture.
When in doubt, ask the local dealer about your engine's age. Dealers have access to other databases and internal information that isn't available to the general public. The Master Tech Marine link has links to lists for other old manufacturers in addition to Mercury outboards.