Starting around 1980 dirt bikes sold in the USA had to conform to the same VIN numbering scheme that automobiles use. Most of the digits are coded to indicate key information. For example, the first letter is the country of manufacture (J= Japan in this case), the second letter identifies the manufacturer (Y =Yamaha).
The 10th digit is the key for the year. The system was set up for starting in 1971, so "1' could equal 1971. After 1979 it goes to letters, so "A" would be 1980. They skip the letters that could easily be confused for numbers or another letter, so I, O, Q, U, and Z are not used. Thus, "Y" is the code for 2000. Starting in 2001 the codes start over, so 1 could be 1971 or 2001 (hopefully you can tell the difference....)
Rod