Manco started to use the "thunderbird" name in the later 1980's when they took over the line from Bird/Phoenix. They used that name for like 20 years. Judging by the decaling and recoil you have on the picture you posted up I would say its around a 1994 built bike. look for stamping on the top of the shroud and look at the last group of numbers on the right, If it is not stamped the look for a decal on the lower back side of the shroud (under the throttle assy) look for the DOM code.
the first digit in either that last group of numbers from the stamp or the DOM is the year it was built. The three numbers following it are the day of the year it was made.
Example: 3001xx = January 1st 1993. That gets you pretty close to the time the bike was finished by Manco and ready to sell.